* Alice calls Bob; Bob's Gabble sends a <ringing/> notification.
* Bob puts the call on Hold; his Gabble sends a <hold/> notification.
* Bob answers the call; his Gabble sends <active/> which cancels the <hold/>.
So now Alice thinks she's off hold, but Bob still thinks the call's on hold.
I'm inclined to say that we don't want to support answering calls while they're on hold (it's meaningless), and smcv thinks this makes sense. So maybe AddMembers should fail with o.fd.T.StreamedMedia.OnHold or something.
As of 0.7.29, this fails with NotAvailable.