### Answer To integrate your Telnyx number with Retell AI and connect it to your external PBX, you'll use Elastic SIP Trunking [(1) ]() [(2) ]() . Here's the step-by-step process: 1. Create Elastic SIP Trunk in Telnyx [(1) ]() Create a trunk in Telnyx, select FQDN as the type, and give it a name . Add Retell's SIP server FQDN:

sip.retellai.com

. Select

SRV

as the DNS record type . 2. Configure Outbound Authentication Select credentials as the authentication method and add a username and password . You'll need these credentials when importing the number to Retell . Note that Telnyx requires the header

X-Telnyx-Username: <username>

for outbound calls when using credentials . 3. Configure Inbound Settings - Select

+E.164

as the number format - Select

G722

,

G711U

,

G711A

as the codecs - Select

TCP

as the transport method (TCP is recommended over UDP due to reliability) - Select your SIP region 4. Configure Outbound Settings Create a new outbound voice profile and select it in the outbound setting . 5. Move Numbers to the Elastic SIP Trunk Purchase numbers or move existing numbers to this trunk in Telnyx . 6. Import Number to Retell AI You'll supply Telnyx's FQDN as the termination SIP URI (find your FQDN based on your SIP region, e.g., sip.telnyx.com) . You'll also need to supply the username, password, and any additional SIP headers you set up in outbound authentication . You can import programmatically via the Import Number API [(3) ]() or through the Retell dashboard [(1) ]() . Once imported, the number will show up in your dashboard and can make and receive calls just like a Retell

-purchased number . Transport Methods Supported: [(2) ]() - TCP (Recommended) - UDP - TLS - mTLS Retell SIP Server Details: - URI:

sip:sip.retellai.com

- IP block for traffic:

18.98.16.120/30

(All regions) ,

143.223.88.0/21

(certain US traffic) ,

161.115.160.0/19

(certain US traffic)

​ To learn more, see the sources I considered: [[1] Telnyx](https://docs.retellai.com/deploy/telnyx) [[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.