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

[Colin Anderson]

>Turn off voicemail on his cell phone, give out his DID instead of his cell
#. 
>Send an SMS to his cellphone when new voicemail is left. 
That's what we do now. Works fine. 

>As far as Dial()ing his cell goes, use 'r' (this is exactly what it's
designed
>for) so that when the carrier is saying "The person  you're calling is out
of
>the calling area or has his phone off" all the caller hears is ringing.

That appears to work *perfectly* but I don't get it. With the 'r' option on,
how can Asterisk determine that the user has answered the phone as opposed
to the carrier? Is it a signal that the carrier is sending?
Anyway, thanks. Works like a hot damn.