Nowadays, if you want to make an audio/video connection using XMPP protocol, you need at least one of the peer to have a real IP with the ports open, or to traverse several IP ports though NAT.
If both peers are behind NAT, audio/video connection will not succeed. Of course, users can use STUN proxy, but this is too complicated to setup, and, users need to know addresses of the STUN servers, which is also not obvious.