On mobiles you can dial *# and numbers and send them off to the network to get back your credit and things like that. Is there anything interesting you can do on a SIP phone to query & reply stuff?
One thing for instance would be account status, so *#222# or whatever would bring back a +10 or -2 or whatever your SIP account status is.
Obviously this depends very much on the client used to connect to the network, but if my phone can say "PSTN access denied" when I try and dial, is it not possible to feed it more info like that?
Just thinking aloud.
Cheers
Daniel