Does anyone know why a Google Apps account can't make or receive GTalk calls, but is working fine with GTalk IM?

I have a Google Apps account that won't make or receive calls from within the Google Talk desktop app, so it's not an N950 issue. If I call another party, I hear the phone ring for a split second then it disconnects and the app claims the other party did not answer.

Other Google Talk accounts (not Google Apps, just regular @gmail.com accounts) on the same PC can make/receive calls to/from the same "other" parties, so it's not my PC setup or the parties being called. It's as if the Google Apps account is screwed up somehow with respect to making/receiving calls (calls are not working on the N950 either, with similar call dropping behaviour, while IM is fine, which tends to points to a backend server config issue). All of which is a shame, as it's my public account so I'd like to get it working...

The Google Apps account has got the Google Talk service assigned to it in the Google Apps dashboard, and IM is working fine. Other people with N950s and Google Apps accounts are making/receiving calls no problem, so the fact it's a Google Apps account shouldn't be an issue, unless I've forgotten to enable something (can't think what).

Anyone have any suggestions as to what could be wrong? I have Googled extensively, but my Google-fu has let me down.

Many thanks in advance, I'm at my wits end...