asterisk users - Jan 2008 - Dialplan flow on device state change

Hello everybody,
is it possible to execute some dialplan code when an extension changes its
state?

Say that SIP/204 becomes busy. I know that using the hint priority some phones
can show its state. Am I able to call custom code in the same way?

Thanks in advance,

-- 
Dr. Andrea Spadaccini
Multimedia Technologies Institute - MTI S.r.l.
Web: www.x-voice.it - Tel: +39 (0) 95 7224945