### Answer When inbound calls fail with "The number you have dialed is incorrect," this typically indicates an issue with your Twilio Elastic SIP Trunk's origination settings. [(1) ]() [(2) ]() For inbound calls to work properly, you need to specify Retell's SIP server address as the origination SIP URI:

sip:sip.retellai.com

. [(1) ]() If you're using a specific transport method, append it to the URI (e.g.,

sip:sip.retellai.com;transport=tcp

for TCP,

sip:sip.retellai.com;transport=udp

for UDP, or

sip:sip.retellai.com;transport=tls

for TLS) . [(2) ]() Common troubleshooting steps: 1. Check your origination setting in your SIP trunking provider - ensure it points to

sip:sip.retellai.com

[(1) ]() 2. Verify your termination SIP URI has no spaces and uses a localized URI near your region 3. Confirm IP whitelisting - You need to whitelist Retell's SIP SBC CIDR block

18.98.16.120/30

4. Review authentication - If using credentials, ensure the username and password match what's shown in Twilio (note: the username is different from the friendly name) 5. Check your telephony provider logs and consider opening a ticket with them if the configuration appears correct [(2) ]() After verifying your Twilio trunk configuration, make sure you've properly imported the number into Retell with the correct termination URI and any required authentication credentials. [(3) ]()

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[3] Import Phone Number](https://docs.retellai.com/api-references/import-phone-number) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

@AI Evy I set the origination is transport type compulsory https://cdn.discordapp.com/attachments/1510936062648651788/1510936990168645683/Screenshot_2026-06-01_at_09.22.24.png?ex=6a1ea13a&is=6a1d4fba&hm=c2540beaced7a920449088bc58fba4b0712af928e9d30726ddcf95f3beab9889& https://cdn.discordapp.com/attachments/1510936062648651788/1510936990491611176/Screenshot_2026-06-01_at_09.23.22.png?ex=6a1ea13a&is=6a1d4fba&hm=6848e3c62d1dd6a73cdb11b828f58010cd887d62d029542bbda7a6e792bff9d1&

### Answer The transport type is not strictly compulsory for the origination URI, but specifying it can help ensure proper routing. [(1) ]() For Twilio inbound calls, you can use the base URI

sip:sip.retellai.com

without specifying transport, and the system will handle it. However, if you want to explicitly set the transport method, you can append it to the SIP server URL: - For TCP:

sip:sip.retellai.com;transport=tcp

- For UDP:

sip:sip.retellai.com;transport=udp

- For TLS:

sip:sip.retellai.com;transport=tls

TCP is the recommended transport method due to reliability factors. [(2) ]() Since you're still experiencing the "incorrect number" error even after setting the origination, double-check these additional items: 1. Verify the origination setting in your Twilio SIP trunk points exactly to Retell's SIP server address [(1) ]() 2. Check the logs in your telephony provider for more specific error details 3. Consider opening a support ticket with Twilio if the configuration appears correct The error message you're receiving typically indicates a routing issue on the telephony provider side rather than a transport protocol issue.

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Telnyx](https://docs.retellai.com/deploy/telnyx) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer I understand your frustration. Since your origination is correctly configured, let's focus on other potential issues that could cause inbound call failures: Check these specific settings: 1. IP Whitelisting in Twilio - Ensure you've whitelisted Retell's CIDR block

18.98.16.120/30

in your Twilio trunk's origination settings [(1) ]() [(2) ]() 2. Number Assignment in Twilio - Verify the phone number is actually assigned to your Elastic SIP Trunk in Twilio's console [(2) ]() 3. Import Configuration in Retell - When you imported the number into Retell, confirm you provided the correct termination URI (should end with

.pstn.twilio.com

) [(3) ]() [(2) ]() 4. Agent Binding - Verify the inbound agent is properly bound to the imported number in Retell's dashboard [(3) ]() 5. Authentication Credentials - If you set up SIP trunk authentication with username/password, ensure those exact credentials were entered when importing the number into Retell (note: the username is NOT the friendly name shown in Twilio's credential list) [(2) ]() Next debugging step: Check your Twilio logs for the specific error code when an inbound call fails - this will show whether the call is reaching Twilio's trunk but failing to route to Retell, or if it's failing before reaching the trunk entirely [(1) ]() [(2) ]()

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Twilio](https://docs.retellai.com/deploy/twilio) [[3] Import Phone Number](https://docs.retellai.com/api-references/import-phone-number) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer To whitelist Retell AI in your Twilio Elastic SIP Trunk, you need to add Retell's SIP SBC CIDR block to your trunk's termination settings: [(1) ]() [(2) ]() Steps to whitelist Retell AI in Twilio: 1. Go to your Elastic SIP Trunk in Twilio console 2. Navigate to the Termination section 3. Under IP Access Control Lists, add the following CIDR block: - CIDR:

18.98.16.120/30

Additional IP ranges to whitelist: [(2) ]() -

143.223.88.0/21

(certain United States traffic) -

161.115.160.0/19

(certain United States traffic) This whitelisting allows your Twilio trunk to accept outbound requests from Retell's SIP servers. [(1) ]() If you prefer authentication via credentials instead of IP whitelisting, you can set up a username and password in Twilio's credential list and provide those when importing the number into Retell.

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer I can't see the screenshot you're referring to, but I can guide you through the correct whitelisting setup for Retell AI in Twilio. For your Twilio Elastic SIP Trunk termination settings, you need to whitelist Retell's CIDR block

18.98.16.120/30

. [(1) ]() Here's what you should verify in your Twilio console: 1. Navigate to your Elastic SIP Trunk 2. Go to the Termination section (this is for outbound calls from Retell) 3. Under IP Access Control Lists, you should have an entry with: - CIDR:

18.98.16.120/30

Important note from recent updates: There was a discussion in the Discord community about whether to keep old ACLs or replace them. If you previously had ACLs with ranges like

0.0.0.0/1

and

128.0.0.0/1

(which covers all IPs) , you should replace those with just the specific Retell CIDR block

18.98.16.120/30

. [(2) ]() Additional CIDR blocks for comprehensive coverage: [(3) ]() -

143.223.88.0/21

(certain United States traffic) -

161.115.160.0/19

(certain United States traffic) If you can describe what you see in your Twilio termination settings or share the specific configuration, I can help confirm if it's set up correctly.

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] Discord message from @dhruvil_49667 on 2025-11-20](https://discord.com/channels/1186424376300679209/1186462578310910112/1441002697565274122) [[3] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

@AI Evy So since when do users have to add whitelisted ips to succesfully make inbound calls to connected number?

### Answer I need to clarify an important distinction about IP whitelisting: IP whitelisting is specifically required for outbound calls from Retell, not inbound calls to Retell. [(1) ]() When you whitelist Retell's IP addresses in your Twilio termination settings, you're allowing Retell to make outbound calls through your Twilio trunk. The termination setting controls traffic going out from Retell to the PSTN through Twilio. For inbound calls (calls coming into your number and reaching Retell) , the critical configuration is the origination setting in your Twilio trunk, which should point to

sip:sip.retellai.com

. The FAQ in the documentation specifically addresses inbound call issues: "My inbound call is not connecting, what should I do? Please check your origination setting in your SIP trunking provider." Based on the sources, IP whitelisting has been part of the Elastic SIP Trunking setup documentation, but it's for the termination (outbound) direction. Your inbound call failure with the "incorrect number" message is more likely related to: 1. Origination configuration pointing to Retell's SIP server 2. The number being properly assigned to the trunk in Twilio 3. The number being correctly imported into Retell with proper termination URI If inbound calls were working previously without IP whitelisting and suddenly stopped, I'd recommend checking your Twilio logs for specific error codes to identify what changed.

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer To check Twilio logs for failing inbound calls: Access Twilio Debugger: 1. Log into your Twilio Console 2. Navigate to Monitor → Logs → Errors or go directly to the [Twilio Debugger]() [(1) ]() 3. Filter by your phone number to see call-specific errors Check Call Logs: 1. Go to Monitor → Logs → Calls in your Twilio Console 2. Look for recent inbound attempts to your number 3. Click on individual call SIDs to see detailed error information What to look for in the logs: - Error codes (e.g., 32022 indicates "ACK not received from your SIP endpoint") [(2) ]() - SIP response codes - Routing failures - Configuration mismatches The Retell documentation specifically recommends: "Check the logs in your telephony provider, and perhaps open a ticket with them" when troubleshooting inbound connection issues. [(3) ]() Common error indicators: - If logs show the call isn't reaching your SIP trunk at all, the number may not be properly assigned to the trunk in Twilio - If logs show SIP connection failures, verify your origination URI configuration - If logs show authentication errors, check your credential setup After reviewing the logs, if the configuration appears correct but calls still fail, consider opening a support ticket with Twilio with your specific error codes and call SIDs for deeper investigation. [(3) ]() [(1) ]()

​ To learn more, see the sources I considered: [[1] Inbound Connection Issue](https://discord.com/channels/1186424376300679209/1345040201604661350) [[2] Discord message from @Deleted User on 2025-05-23](https://discord.com/channels/1186424376300679209/1186462578310910112/1375385554996166687) [[3] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer If there are no logs in Twilio for the last 24 hours, this suggests the calls may not be reaching your Twilio trunk at all. This points to a potential issue with how the number is configured in Twilio itself, rather than the SIP trunk routing. Verify these critical settings: 1. Number Assignment in Twilio - Confirm the phone number is actually assigned to your Elastic SIP Trunk in Twilio's console [(1) ]() 2. Number Status - Check if the number is active and properly provisioned in Twilio 3. Carrier-Level Issues - If calls aren't generating any logs, the issue may be at the carrier level before reaching Twilio Alternative debugging approaches: - Try calling the number from different phones/carriers to see if the issue is carrier-specific - Check if the number works when temporarily configured with a different Twilio service (like a TwiML Bin) to isolate whether the issue is with the trunk or the number itself - Verify the number format is correct (E.164 format: country code + number without spaces) [(2) ]() Recent platform issues to consider: There was a documented incident on October 28, 2025 where "inbound calls were experiencing connection issues and timeouts" due to "a brief outage with our telephony provider." [(3) ]() While this was resolved, if your issues started recently, it's worth checking Retell's status page for any ongoing incidents. Next steps: 1. Contact Twilio support directly with your account SID and phone number - the absence of logs is unusual and they can investigate at the carrier level 2. Try making a test outbound call from Retell using this number to verify the termination (outbound) direction work

s 3. If outbound works but inbound doesn't, and there are no Twilio logs, this strongly suggests the number isn't properly routing to your trunk

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] AI in SIP Calling & Telephony Solutions | Retell AI](https://www.retellai.com/blog/integrating-ai-with-sip-trunking-and-modern-telephony) [[3] Intermittent Call Connection Failures](https://community.retellai.com/t/intermittent-call-connection-failures/143) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] Import Phone Number](https://docs.retellai.com/api-references/import-phone-number) ​

### Answer I understand your frustration. Based on your troubleshooting, let me suggest a systematic verification of the complete setup: Critical verification steps: 1. In Twilio Console - Number Configuration: - Go to Phone Numbers → Manage → Active Numbers - Click on your specific number - Verify "Configure With" is set to "SIP Trunk" and your Elastic SIP Trunk is selected - If it's set to anything else (TwiML, Function, etc.) , the number won't route to your trunk at all 2. In Twilio - Elastic SIP Trunk Origination: - Confirm the Origination SIP URI is exactly:

sip:sip.retellai.com

[(1) ]() - No extra characters, spaces, or incorrect transport parameters 3. In Retell Dashboard - Number Import: - Verify the number was imported with the correct termination URI (should end with

.pstn.twilio.com

) [(1) ]() [(2) ]() - Confirm an inbound agent is bound to this number [(2) ]() 4. Test with a fresh number: - Purchase a new Twilio number - Assign it to your existing trunk - Import it into Retell - Test inbound - this isolates whether the issue is number-specific If none of this works: Since there are no Twilio logs appearing at all, this strongly suggests the calls aren't reaching your Twilio account. Contact Twilio support with: - Your account SID - The specific phone number - Timestamp of test calls - Request they trace the inbound routing path You can also reach out to Retell support at support@retellai.com with your configuration details and Twilio findings for direct assistance with your specific setup.

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

ur account SID and request they trace the inbound routing for these specific numbers. The absence of any logs suggests calls aren't reaching your Twilio account at all, which points to a carrier-level or number provisioning issue that only Twilio can diagnose. [(2) ]()

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[3] Elastic SIP Trunk Busy Signal](https://discord.com/channels/1186424376300679209/1415758706133696562) [[4] Discord message from @onrise_jb on 2026-03-10](https://discord.com/channels/1186424376300679209/1186462578310910112/1480958902228091044) ​

### Answer Since all your numbers are showing this exact error message, this indicates a systematic configuration issue rather than a number-specific problem. The "incorrect number" message typically comes from the carrier/PSTN level before the call even reaches Twilio. Critical check - Number assignment in Twilio: In your Twilio console, for each number: 1. Navigate to Phone Numbers → Manage → Active Numbers 2. Click each number individually 3. Under "Configure With", verify it shows "SIP Trunk" and your specific Elastic SIP Trunk is selected [(1) ]() 4. If any number shows "TwiML App", "Function", "Studio Flow", or anything other than your SIP Trunk, the calls won't route to Retell at all Verify Twilio trunk origination format: Your origination SIP URI must be exactly:

sip:sip.retellai.com

For specific transport methods, use: [(2) ]() - TCP:

sip:sip.retellai.com;transport=tcp

- UDP:

sip:sip.retellai.com;transport=udp

- TLS:

sip:sip.retellai.com;transport=tls

Similar issue from Discord: A user experienced inbound busy signals with the correct origination URI

sip:5t4n6j0wnrl.sip.livekit.cloud

and was advised to add the transport parameter:

sip:5t4n6j0wnrl.sip.livekit.cloud;transport=tls

[(3) ]() Number deletion/re-import issue: There's a known issue where deleted numbers cannot be re-added, showing "Phone number already exists" error. [(4) ]() If you've deleted and tried to re-import any of these numbers, this could be blocking proper configuration. Immediate action: Since no Twilio logs are appearing and ALL numbers fail identically, contact Twilio support directly with yo

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer This symptom—where you hear "number isn't available" but Retell logs show the agent answered and detected silence—indicates a split routing issue. The call is reaching Retell's platform (hence the logs) , but the audio path back to you is failing, so you hear a carrier error message instead of the agent. Key diagnostic finding: The fact that Retell logs show the agent responding with "user said nothing and dropped call" confirms the SIP connection to Retell is working. The problem is the bidirectional audio path isn't completing properly. Likely causes: 1. Codec mismatch - Your Twilio trunk and Retell may be negotiating incompatible audio codecs, causing one-way audio 2. NAT/Firewall issues - RTP (Real-time Transport Protocol) packets carrying audio may be blocked in one direction 3. Twilio trunk termination misconfiguration - The return path from Retell back through your trunk may not be properly configured Critical verification steps: In Twilio - Check both directions: - Origination (inbound to Retell) : Verify

sip:sip.retellai.com

or with transport parameter - Termination (outbound from Retell) : Ensure Retell's CIDR

18.98.16.120/30

is whitelisted in your IP Access Control List In Retell - Verify number import: - Confirm the termination URI in your imported number configuration ends with

.pstn.twilio.com

- Check if authentication credentials (if using username/password instead of IP whitelisting) are correctly entered Test the reverse direction: Make an outbound test call from Retell using this same number. If outbound works but inbound doesn't, this confirms the termination (outbound) path is correct but origination (inbound) has the audio routing issue. Immediate troubleshooting: Contact Twilio support with: - Your account SID - Specific call SIDs from Retell logs showing t

he "silence" calls - Request they analyze the SIP signaling and RTP flow for these calls - Ask them to verify codec negotiation and bidirectional audio path The combination of carrier error message + Retell logs showing connection suggests a SIP signaling success but media (audio) failure scenario that Twilio can trace at the protocol level.

message has been deleted

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.