We are having an identical issue. Have not been able to determine the nature of the issue as of yet. T-mobile has blamed the phone system, the trunk and a multitude of other things, and I can't find anything about the way the system dials that would cause such a problem. My next try is going to be to try to alter the called and redirect number before it leaves the gateway, perhaps T-Mobile doesn't like the way they are being presented.
-S