Works OK with an GrandStream PBX

The setup up is your name, the IP of the PBX (the app calls domain), username/password.
To call type in number. To answer push accept.
Thats the entire UI - There is no dial pad, volume control - its as basic as one can get.
First phone either virtual or physical that does not have dial pad that I have ever seen. So this means the phone does not have access to any options your PBX or SIP service may offer, like voicemail.
I can answer / make calls on it - that it.

Doug77777 about Telephone