I just sent and received calls between my SIP account and landline phone using my #Librem5 and the native Calls application, and it was pretty exciting.
@kyle gonna play with sip and gnome-calls on desktop =D
I've had success with linphone.org, jmp.chat, sipgate.de and my local ISP.
For my own ISP I actually don't have telephony set up at all in my router and instead used the SIP credentials directly in Calls.
I have to try emulating that setup on my end and see if I run into the same error.
What strikes me as odd is that registering seemed successful?!
For debugging you can also set TPORT_LOG=1 which will print all sent/received SIP messages to stdout.
Also feel free to hit me up on the Calls matrix channel, so we can try getting to the bottom of this :)
Hm, this is funny - I had a call while not at my computer and when I returned it showed me a gnome notification about the missed call (after reboots gnome-calls had been running as a daemon I found).
Still I can't call out, but the registration seems to work this far that gnome-calls gets notified about incoming calls :).