### Answer Based on the Retell AI documentation, you have two main approaches to connect your Aircall number to a Retell agent [(1) ]() : ## Method 1: Elastic SIP Trunking (Recommended) This is the recommended approach where you configure Aircall to forward calls to Retell's SIP server . You would: 1. Set up SIP trunking in Aircall 2. Point your Aircall number to Retell's SIP server:

sip:sip.retellai.com

3. Configure the transport method (TCP recommended) :

sip:sip.retellai.com;transport=tcp

This method supports all Retell telephony features including call transfer . ## Method 2: Dial to SIP URI If Aircall doesn't support elastic SIP trunking or you need more control, use the API approach : 1. When Aircall receives a call, trigger a webhook to your server 2. Call the Register Phone Call API to get a

call_id

[(2) ]() :

javascript
const phoneCallResponse = await client.call.registerPhoneCall({
 agent_id: 'oBeDLoLOeuAbiuaMFXRtDOLriTJ5tSxD',
 from_number: "+12137771234",
 to_number: "+12137771235",
 direction: "inbound",
}) ;

3. Use the returned

call_id

to construct the SIP URI:

sip:{call_id}@sip.retellai.com

[(1) ]() 4. Have Aircall dial this SIP URI to connect the call to your Retell agent **Note**: With Method 2, you'll need to implement your own transfer logic using custom functions, as Retell won't have direct control over the telephony provider . Check if Aircall supports SIP trunking first—if yes, Method 1 is simpler and more feature-complete.

​ To learn more, see the sources I considered: [[1] Custom Telephony Overview](https://docs.retellai.com/deploy/custom-telephony) [[2] Register Phone Call](https://docs.retellai.com/api-references/register-phone-call) ​

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