Same here - i have just installed FreePBX (docker) as i’m looking to use it for my doorbell.
Using this intercom doorbell (with SIP)
Right now if someone pushes the doorbell button I could grab the MQTT alert using this and send an actionable alert to phones, or browser_mod.popup to tablets.
Issue is it isnt a ‘call’ and not only would my only option be to maybe open the door, I wouldnt be able to talk back. My thinking is to use a SIP client on my phones and initiate a call when the doorbell is pressed (i could use tasker and MQTT to do the same on the wall tablets) but it would all be easier if there was a SIP or PBX integration of some kind.