[Telepathy] Make Google Hangouts voice and video chat work (Ubuntu 14.04)

[Peter Bittner]

I've got an Ubuntu 14.04.3 LTS box running Empathy 3.8.6, and I've got
a Google (apps) account configured.

According to the documentation [1] voice and video chat should work
with Google, provided GTalk corresponds Hangouts today. I've checked
the installed codecs, and the ones listed in the docs [1] are
installed.

Now, I do have Voice Call and Video Call in the user's context menu,
but for most users these options are grayed out. Not sure why: One
contact I called today was using their Google (Hangouts) account on a
Windows box and Psi messenger, another one is using their Google apps
account (i.e. custom domain, as I do) and is online now with Empathy.
(Interestingly, the former has the two options grayed out again now
that the account is set to "Away". Strange!)

Is there any specific rule to when voice and video chat is available?
Peter

[1] https://wiki.gnome.org/Apps/Empathy/FAQ#Audio_and_Video_calls