SIP trunking clarification for inbound vs outbound authentication

Support Help
1

Hi, I’m currently setting up a SIP integration between a telephony provider and Retell, and I wanted to clarify the expected authentication flow to make sure I’m aligning things correctly.

The provider is saying that since they are sending calls to Retell, Retell should generate SIP credentials (username/password) and share them for configuration on their side.

From my understanding, Retell does not generate SIP credentials, and the typical flow is:

  • For inbound calls (provider → Retell): calls are routed to sip:sip.retellai.com with IP whitelisting, without credentials

  • For outbound calls (Retell → provider): the provider generates SIP trunk credentials, which are then used in Retell

Could you please confirm if this is the correct setup? Also, is there ever a case where Retell would generate credentials for inbound authentication?

Thanks!

2

Hello @r0wan.muhammed0.o

Could you please share the call ID along with the PCAP file for the call? This will help our team investigate the issue in detail.

Thank you!

3

Hi, thanks for the response!

This is not related to a specific call issue, I’m just trying to confirm the correct SIP trunking setup and authentication flow.

From my understanding:

-- Retell does not generate SIP credentials
– Inbound calls (provider → Retell) are handled via routing to sip:sip.retellai.com with IP whitelisting only
– For outbound (Retell → provider), the telephony provider either provides credentials or whitelists Retell’s IP

Could you please confirm if this is correct?

Also, just to clarify a couple of points:

-- Is it correct that inbound traffic to Retell never uses SIP credentials under any setup?
– Is IP whitelisting alone fully sufficient for outbound, without credentials?
– And to confirm, the termination URI is always provided by the telephony provider, not Retell?

Thank you!

4

Hey @r0wan.muhammed0.o

Retell does not generate SIP credentials . Credentials (if any) come from your telephony provider.

Inbound (provider → Retell): Routed to sip:sip.retellai.com. The provider should whitelist Retell’s IP block (e.g., 18.98.16.120/30). No SIP credentials are used on Retell’s side for inbound .

Outbound (Retell → provider): You can either whitelist Retell’s IP block or use username/password auth configured on your trunk. Either alone is sufficient (for Twilio : “you need to whitelist IP address or create an auth with username and password”).

Termination URI: Always provided by your telephony provider (you supply it to Retell when importing the number).

Thank You

5

Thanks a lot for the clear explanation, this really helped!

I’ll proceed with the setup accordingly. Really appreciate the help!