depending on how many people are using it, and how everything is set up... consider this.
you're getting a call, tying up a trunk, that person interacts with the AA, etc. they're still only tying up 1 trunk. now the system tries to forward them off premises. is there an available trunk to make the outbound call? (maybe) can the AA or VM system actually pick up an outbound trunk (maybe, toll fraud.... ) even if they have DID numbers that customers dial into and it skips your local AA entirely, it STILL has to have an open line to make the call out to the cell phone. and depending on your service setup, may require that both trunks sit there busy the entire time the call is in session (I know on the all-analog system I work with that's the nature of the beast)
Bottom line, it may be trying, but not be able to tell you "hey, I can't transfer this call because I don't have a trunk to transfer it TO"
and I could be totally wrong.
