[Asterisk-Users] Looking for advice on cell carrier's default "Un avaliable" message

[Colin Anderson]

In our dialplan, we use centralized voicemail for SIP, IAX and cell phones.
This means, if a caller calls a user's DID, it tries his SIP/IAX extension,
then if he doesn't answer there, it tries his cell, then it goes to Comedian
Mail. 

Everything works 100%, except when the user shuts his cell phone off. When
that happens, and he doesn't pick up his SIP/IAX extension, it hits his cell
phone, and the cell carrier's default Unavailable message is played.
Asterisk detects this as the call being "answered" and completes the call.
However, this is undesirable behavior. We want it to go to Comedian mail
instead. Note that this is contrary to what the carrier said would happen.
The carrier indicated to us that it would just ring and ring and ring
forever, which is what we want. Now they are saying: "too bad, this is the
way it works, deal with it" 

In order to have the desired behavior, there are three options:

1. Carrier makes it ring forever (not gonna happen)
2. I set the call forward/Unavailable on the cell to a DID that points to
Comedian Mail and do some Caller ID stuff to make it go to the right
mailbox. This isn't practical from a management standpoint, it would be
troublesome and error prone to maintain
3. When the cell is off, the carrier's Unavailable message plays right away,
within 2 seconds of the call being dialed. So, somehow magically modify the
dialplan so that if a cell is answered within 2 seconds, go to Comedian
Mail. 

Of these options, 3) would provide the optimum workaround, but I don't think
it's possible to express this in an Asterisk dialplan.

 Anyone have any advice or dialplan magic on how to do 3) ? ?