Linphone supports ICE and therefore it should work more reliably behind NAT than some of the other softphones.
It receives calls from Lumicall users.
Complete the setup wizard
Add some settings manually
Select TLS, select ICE and put in the STUN/TURN server name turn.debian.org
The first time it tries to connect to the SIP proxy, it will complain about authentication.
This is because you have to add @debian.org to the authentication username:
It should remember that and will not prompt again.