please be aware had issue getting these working on PIX/ASA the issue was related to the version of the Unit, 7.2 and above worked everytime, below this software we had various issues.
if you open the right ports / fix ups or whatever they are called now
no fixup protocol h323 h225 1720
no fixup protocol h323 ras 1718-1719
nofixup protocol tftp 69
the iprouter in the system is fine and the phone is setup according the the manual they work grea