Is sipgate (Germany) supported as a SIP trunking provider for Retell, given that sipgate requires SIP REGISTER authentication?

Support Help
1

Hi Retell Support,

I’m trying to connect a German sipgate trunk to Retell via SIP trunking, but the trunk shows as “offline” in my sipgate dashboard and I’m not sure how to proceed.

Setup:

  • Telephony provider: sipgate (Germany), “trunking 2 free” tariff
  • Phone number: +4915792505921 (E.164 format)
  • Termination URI configured in Retell: sip:sipconnect.sipgate.de
  • Outbound Transport: TCP
  • Username/Password from sipgate trunk entered

Issue:
The trunk status in sipgate stays “offline” because sipgate uses classic SIP REGISTER authentication, while Retell (as I understand) uses Elastic SIP Trunking without REGISTER. Outbound test calls also fail.

Questions:

  1. Is sipgate trunking known to be compatible with Retell, or is REGISTER-based authentication a hard blocker?
  2. If not directly compatible, do you officially recommend a bridge solution (e.g., jambonz) for German sipgate users, or should I switch to Twilio/Telnyx?
  3. Are there any sipgate-specific configuration steps in your docs that I might have missed?

Happy to share Retell logs or call IDs if that helps with debugging.

Thanks,
Samuel Kühne
Diginauts Media GmbH

2

Hello @retell1

Could you please share the call IDs along with their PCAP files? This will help our team investigate the issue more effectively.

Thank you!

3

Hi,

There are no call IDs because the calls never reach Retell – the
connection between sipgate and Retell is not established at all.

sipgate uses SIP REGISTER authentication and shows the trunk as
“offline” in their dashboard, since Retell does not register with
sipgate’s server.

Is sipgate supported at all, or is REGISTER-based authentication
incompatible with Retell? If incompatible, what do you recommend
for German numbers?

Thanks,
Samuel

4

Hello @retell1

Retell uses IP-based authentication for SIP trunking (Elastic SIP trunking with whitelisted IPs), not REGISTER-based authentication. Sipgate’s REGISTER-based trunk is therefore incompatible. The supported integrations: Twilio, Telnyx, Vonage, plus telephony partners like Jambonz and Cloudonix that can bridge custom setups.

For German numbers, recommended options :

  • Use a provider supporting Elastic SIP trunking (Twilio, Telnyx, Vonage) — they offer German DIDs.

  • Use a partner like Jambonz or Cloudonix to bridge sipgate’s REGISTER-based trunk to Retell’s IP-based SIP.

  • Alternatively, use Dial to SIP URI (Method 2) where your own system handles sipgate registration and forwards calls to sip:{call_id}@sip.retellai.com.

Thank You