I use Jami for audio calls. On my end, it seems to work well. Seems about as reliable as a Skype was. I’m on an Andoid device. You have to keep it running in the background all the time, if I remember, there was a setting to do so.
The person I talk to is on an iPhone, and they have some issues. From what I can tell (I don’t have an iPhone to test), there are issues on the iPhone with keeping it running in the background. So, when I try to call (from Android phone to iPhone), the call doesn’t go through. When the other person calls (from iPhone to Android) it works fine. I think it’s a known issue with how the iPhone works and handles background apps.
They also have an issue with the iPhone version of the app, where they hit mute, and it somehow gets stuck in a mute-unmute cycle.
tl;dr - on Android Jami has been good, but the iPhone app seems buggy.