### Answer Based on the available documentation, Retell AI does not act as a SIP client that sends SIP REGISTER requests. [(1) ]() Retell provides a SIP server (

sip:sip.retellai.com

) that accepts inbound SIP traffic, but the integration model expects your telephony infrastructure or PBX to initiate calls to Retell's SIP endpoint. The two supported methods are: 1. Elastic SIP Trunking – You configure your telephony provider's SIP trunk to point to Retell, and Retell receives the SIP INVITEs. 2. Dial to SIP URI – You register a call with Retell's API, receive a

call_id

, then dial the generated SIP endpoint (e.g.,

sip:{call_id}@...

) from your own SIP client or PBX. A community member asked a similar question about whether Retell works like a SIP client that sends REGISTER requests to an Asterisk installation. The response indicated that while the SIP trunking integration should work, Retell itself doesn't perform SIP REGISTER—your system needs to handle registration and then send INVITEs to Retell. [(2) ]() If you need SIP REGISTER functionality or more complex SIP call flow manipulation, you may want to consider using a telephony partner like [Jambonz]() , which can sit between your infrastructure and Retell to handle those scenarios. [(1) ]()

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Discord message from @olcay6238 on 2025-02-20](https://discord.com/channels/1186424376300679209/1294891368505937952/1342028471039295559) ​

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

### Answer Yes, you can purchase phone numbers (DIDs) directly from Retell AI without needing to go through Twilio or another provider. Retell offers toll-free numbers for $5/month, and toll-free inbound calls cost $0.06 per minute. [(1) ]() However, if you already have DIDs from an external provider, you'll need to connect them via an Elastic SIP Trunk. [(2) ]() Retell supports providers like Twilio, Telnyx, and Vonage for this purpose. To use external DIDs with Retell: - Set up an elastic SIP trunk with your provider - Configure termination and origination URIs to route calls through Retell's SIP server - Import your phone numbers into Retell in E.164 format You can assign multiple DID numbers to an agent by using the same SIP trunk information for each number. [(3) ]() For purchasing numbers directly through Retell (bypassing external providers entirely) , you'd do that through the Retell dashboard. This avoids the SIP trunk configuration complexity if you don't need to use existing numbers.

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] I’m trying to use a toll-free number (connected from Twilio through a SIP trunk)](https://retell.discourse.group/t/i-m-trying-to-use-a-toll-free-number-connected-from-twilio-through-a-sip-trunk/127) [[2] AI in SIP Calling & Telephony Solutions | Retell AI](https://www.retellai.com/blog/integrating-ai-with-sip-trunking-and-modern-telephony) [[3] Multiple DID numbers](https://discord.com/channels/1186424376300679209/1363817665386385539) ​