I might be totally wrong here but if I understand/interpret your issue correctly, it looks awfully like a situation, where you try to show the initial caller ID on the cell phone instead of the caller ID of your Mitel. In some cases providers block that since you try to forward a number as CLI that is not your's.
What you could check is the following: Look in the "Protocol assignment" of your PRI (I guess T1 for US) and check what the "Replace External CLID" is set to. If it is unchecked, check it and see if the forwarding works.
But as said I might be wrong but I hope I interpreted it correctly.
Dutch