WTF, what happened to the built-in VoIP client? I just wanted to give you a quick overview on how to enable it but couldn't find it. Did they remove it with Android 12 or 13? I'm pretty sure I still had that with Android 11.
Tried ElementX but ran into issues while getting it working with a selfhosted matrix server. After setting up dendrite (matrix server) and sliding-sync server (asynchronous message history syncing; which it requires) I still wasn't able to get past the initial sync screen. Are you hosting your own matrix server, did you use it with a public server, or have you not tried using it yet?
If you arent particular about it being SIP specifically, Signal is fantastic (though not available on F-Droid because the Signal devs are paranoid about not signing their own code - you can get a direct APK from their site).
There is fluffchat which is just client for matrix (an open source chat standard).
i tested it with voice and video and it works great. End to end encryption also. Works on all systems (window, linux, android,iphobe,...). Even thunderbird can work as matrix client.
best of all, just like lemmy you can install your own chat server and have full control
I use linphone sometimes. It's not great but it is usable if you set the codecs to high bandwidth ones. I've been wanting to try Jami but haven't yet bothered.