### Answer Yes, you should whitelist the new CIDR block in your Twilio elastic SIP trunk. [(1) ]() [(2) ]() For Twilio elastic SIP trunking, you need to whitelist Retell's SIP SBC CIDR block

18.98.16.120/30

in your trunk's termination settings. [(1) ]() Additionally, Retell's documentation mentions the IP block

18.98.16.120/30

(All regions) ,

3.42.144.0/23

(All regions) ,

143.223.88.0/21

(certain United States traffic) , and

161.115.160.0/19

(certain United States traffic) for traffic whitelisting. [(2) ]() If you received an email about a new CIDR block, add it to your Twilio trunk's IP Access Control List in the Termination section. This ensures your elastic SIP trunk accepts outbound requests from Retell. [(1) ]() Based on recent Discord discussions, users have been advised to whitelist the new CIDR notation in their IP Access Control List. [(3) ]()

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

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

### Answer Yes, that's the correct section. You need to add the new CIDR block to the IP Access Control Lists under the Authentication section of your Twilio elastic SIP trunk's Termination settings. [(1) ]() According to the Twilio setup documentation, for your elastic SIP trunk to accept Retell's outbound requests, you need to whitelist the IP address. Specifically, you need to whitelist Retell SIP SBC CIDR block

18.98.16.120/30

. The IP Access Control Lists section is where you configure which IP addresses are allowed to send termination calls (outbound from Retell's perspective) to your Twilio trunk. Add the new CIDR block you received in the email to this list. [(1) ]() [(2) ]()

​ To learn more, see the sources I considered: [[1] Twilio](https://docs.retellai.com/deploy/twilio) [[2] Connect Retell voice agents to custom telephony](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 Yes,

3.42.144.0/23

is one of the CIDR blocks you should whitelist for Retell's SIP traffic. [(1) ]() According to the documentation, the IP blocks for Retell traffic include: -

18.98.16.120/30

(All regions) -

3.42.144.0/23

(All regions) -

143.223.88.0/21

(certain United States traffic) -

161.115.160.0/19

(certain United States traffic) If the email mentioned

3.42.144.0/23

specifically, then yes, you should add this CIDR block to your Twilio elastic SIP trunk's IP Access Control Lists in the Termination section. [(2) ]()

​ To learn more, see the sources I considered: [[1] Connect Retell voice agents to custom telephony](https://docs.retellai.com/deploy/custom-telephony) [[2] Twilio](https://docs.retellai.com/deploy/twilio) ​

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

### Answer You should remove the old CIDR blocks

0.0.0.0/1

and

128.0.0.0/1

and keep only the specific Retell IP blocks. [(1) ]() According to recent Discord discussions, users were advised that "the old one is just a range of every ip" and were told to "just [use] the new one" rather than keeping both. When asked whether to remove the earlier ACLs and just add the new CIDR network address, the response was "just the new one." The current CIDR blocks you should have whitelisted are: -

18.98.16.120/30

(All regions) -

3.42.144.0/23

(All regions) -

143.223.88.0/21

(certain United States traffic) -

161.115.160.0/19

(certain United States traffic) [(2) ]() The

0.0.0.0/1

and

128.0.0.0/1

entries essentially allowed traffic from all IP addresses, which is less secure. You should remove those and keep only the specific Retell CIDR blocks listed above.

​ To learn more, see the sources I considered: [[1] Discord message from @dhruvil_49667 on 2025-11-20](https://discord.com/channels/1186424376300679209/1186462578310910112/1441002697565274122) [[2] Connect Retell voice agents to custom telephony](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 No, you should have multiple CIDR blocks whitelisted in your Twilio IP Access Control List, not just one. [(1) ]() According to the Twilio setup documentation, you need to whitelist Retell SIP SBC CIDR block

18.98.16.120/30

. Based on the recent email notification you received, the complete list of CIDR blocks you should have configured includes: -

18.98.16.120/30

(All regions) -

3.42.144.0/23

(All regions) -

143.223.88.0/21

(certain United States traffic) -

161.115.160.0/19

(certain United States traffic) You should remove the old

0.0.0.0/1

and

128.0.0.0/1

entries (which allowed all IP addresses) , but keep all the specific Retell CIDR blocks listed above. [(2) ]()

​ 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) ​

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

### Answer Yes, for UK-based operations, you should whitelist both "All regions" CIDR blocks: -

18.98.16.120/30

(All regions) -

3.42.144.0/23

(All regions) [(1) ]() The documentation lists these as covering all regions, which includes UK traffic. The additional CIDR blocks

143.223.88.0/21

and

161.115.160.0/19

are specifically noted as covering "certain United States traffic," so those would not be necessary for UK operations. Configure both of the "All regions" CIDR blocks in your Twilio elastic SIP trunk's IP Access Control Lists under the Termination section. [(2) ]()

​ To learn more, see the sources I considered: [[1] Connect Retell voice agents to custom telephony](https://docs.retellai.com/deploy/custom-telephony) [[2] Twilio](https://docs.retellai.com/deploy/twilio) ​

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

### Answer Great! That's exactly right for UK operations. You've correctly configured both "All regions" CIDR blocks (

18.98.16.120/30

and

3.42.144.0/23

) , which is what you need for Retell's SIP traffic in the UK. [(1) ]() Since your calls are working properly, your Twilio elastic SIP trunk is now correctly whitelisting Retell's IP addresses for outbound call termination. The configuration should ensure uninterrupted service going forward. [(2) ]() [(1) ]()

​ To learn more, see the sources I considered: [[1] Connect Retell voice agents to custom telephony](https://docs.retellai.com/deploy/custom-telephony) [[2] Twilio](https://docs.retellai.com/deploy/twilio) ​

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