It should have rang your deskphone for the 12 seconds or so as well, at the minute there is a bug where if you turn Human Answer Supervision OFF it doesn't twin properly. It rings exactly as you describe, desk phone for a couple of seconds then the mobile, if you had a forward to voicemail as the final step I don't believe that works either.
There's a setting, I think in the Trunk group called Propogate Original Callers ID, this is supposed to forward on the incoming callers ID to your Cell, this doesn't work in the UK, who knows in Canada!