could be that Use Fritz!Box for outbound Calls and Announce incoming calls are of help as well?
I addition there are, which haven’t been mentioned yet in this thread:
- Answer my door anywhere with SIP call using old android phone/tablet, Bluetooth speaker, Tasker and MQTT
- Doorbird and SIP
and an example how it it is integrated in Loxone using DoorBirds API is shown in https://www.loxwiki.eu/display/LOX/Doorbird (in German)