I know this isn't Mitel related but since we're discussing SIP here I thought I'd share this one.
We have a customer using a Cisco Express and SIP trunks.
Customer was having trouble with calls coming in from certain cell phone providers.
The problem turned out to be the callerl ID.
If the caller ID had a comma (,) in it i.e. Lastname, first name, the call would fail.
The resolution was to downgrade the software in the Cisco. Fauld was with the lastest version.
Ralph