XMPP has the jingle extension for audio and video calls: https://en.wikipedia.org/wiki/Jingle_(protocol)

I have successfully used this for voice calls between the chat clients Gajim/Pidgin (I do not remember which) and Google Talk (the now-discontinued XMPP client by Google). I have also done video calls between two Nokia N900 phones, to see if it works. Voice and video via XMPP has worked since approximately 2009 and is just neglected by the companies for business reasons.

XMPP has the draft extension of Jingle, and there are Jingle extensions for things like file transfer. Similarly SIP/SIMPLE could be used, or WebRTC. Any of the three could be used instead of the mess we have now.

I am not aware of a good obvious default answer. And even when there is a reasonable one (ie xmpp for text chat) it doesn't appear that the open flavor has won in the market place.