Yeah, but most if not all the accounts in question are also "data only" and voice barred, so they would never get a call anyway. I suppose in this case calling them would not be an option even if it did work as a hack.
Perhaps T-Mobile does not populate the phone number on the SIM if the account is to be data only, however we have many others that are populated and that are also voice barred so it is inconsistent...
BESX 4.1.7 on Exchange 2003: 65 Devices
BESX 5.0.3 on Exchange 2003: 2007 Devices