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

### Answer There are two main methods to connect custom telephony providers like Zadarma, Telnyx, and Twilio to Retell: [(1) ]() *Method 1: Elastic SIP Trunking (Recommended) * This is the preferred approach. You need to: 1. Set up SIP trunking with your provider 2. Configure your number to point to the trunk 3. Import that number to Retell Retell SIP server URI:

sip:sip.retellai.com

Transport methods supported: TCP (Recommended) , UDP, TLS For different transports on inbound calls: - TCP:

sip:sip.retellai.com;transport=tcp

- UDP:

sip:sip.retellai.com;transport=udp

- TLS:

sip:sip.retellai.com;transport=tls

IP block for whitelisting:

18.98.16.120/30

(All regions) ,

143.223.88.0/21

(United States only) ,

161.115.160.0/19

(United States only) Method 2: Dial to SIP URI If elastic SIP trunking doesn't work for your setup, use the [Register Phone Call API]() to get a

call_id

, then dial to:

sip:{call_id}@sip.retellai.com

Troubleshooting Tips: - If inbound calls aren't connecting, check your origination settings in your SIP trunking provider - If outbound calls aren't connecting, verify your termination settings and ensure you provided the correct termination URL to Retell - Retell has detailed guides for [Twilio]() , [Telnyx]() , and [Vonage]() Could you share more details about what specific error or behavior you're seeing when trying to connect? That would help narrow down the issue.

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