The setup is like this:
[PSTN]--T1 PRI--[CUCM MGCP G/W]--T1 PRI--[Mitel PBX]--[analog phones]
A person has a phone number that they don't realize is long distance, so they dial 9 250 xxx-xxxx. If he does it on a Cisco IP phone, he gets a recording saying that the call cannot be completed as dialed. However, if dialed on a Mitel phone, they get nothing at all, just dead air for about ten seconds, before it starts to do the rapid busy signal sound.
Any idea how this's fixed?