At work, I currently face the challenge of finding a Linux softphone I can use with a 3CX PBX.
With sipgate we used Linphone, which worked better than sipgate's Softphone or CLINQ apps. But with 3CX the best I could achieve is registering the softphone. With Linphone I can initiate calls but as soon as I pick up the connection gets terminated by the remote side again (at least that's what Linphone says).
I tried Blink, which at least doesn't terminate immediately. But I don't get any sound, on neither end. I can hear recordings originating from the PBX, though. The codec negotiation works, too, as I went through each codec individually.
With Jami and GNOME Calls I can't even register the softphone with the PBX.