Is there a link to this I can’t find?

All the documentation is really scattered (partially due to the interwoven nature of having a stack of different protocols/services).

necessary for outbound telephone calls (to landline/mobile phone network)?

… or, put another way:

• are there any XMPP clients one is aware of which can NOT do so?

More generally, Is there a list of What XMPP extensions are used by JMPchat (and for which purposes)?