we talked about this before (I think we talk about VoIP across many threads?) at VOIP App on e/OS - #9 by tcecyk - in theory it should work when the device is registered and Google Cloud Messaging (GCM) in microG settings enabled.
I use a proprietary VoIP service that makes use of GCM. Now my phone is very often in do-not-disturb mode, but when it’s on “ringing” I generally do not miss incoming VoIP calls. What I’m trying to say: did you ever test Acrobits on its incoming behaviour yourself after explicitly enabling device registration and GCM (it shoul be enabled by default anyway) ?
I have no experience with ntfy / UP and Jami so can’t add to this. But to answer if you can use it with any client: the client needs to support unified push to do so.