[tsac]

As aiharkness said the 9900 requires a BIS data connection. You indicate the service is for a Q10 and works ok. The carrier does not implement a specific data plan that required the BIS connection for the secure type plan offered on the BIS and BES data connections. Switching from the Q10 to the 9900 on the same line/plan does not work because the carrier routes the data via the server equipment based on the phone. They also may filter some services as you found trying to switch.
BIS offered data compression and other features not available on the non-BIS service. Most Carriers no longer offer unlimited data plans so using a non compression service is to their advantage. You use more for the same message. Some carriers still offer BIS service on the BB10 OS models but I doubt any will allow the switching of the two type phones without calling or notifying them.