said by MRCUR:Ahh. Ok. Thanks for explaining.
The current GSM model iPhone 5 (sold by AT&T and by Apple as the unlocked model) can work on AT&T and T-Mobile's LTE networks. While the phone can work on AT&T's HSPA+ network, it cannot work on T-Mobile's (which uses AWS for both HSPA+ and LTE, AT&T only uses AWS for LTE along with 700b).
Because of this, the current GSM iPhone 5 (model A1428) has been running just fine on T-Mobile's EDGE network, and on HSPA+ where T-Mobile has reallocated 1900MHz spectrum to serve HSPA+. Otherwise, the current phone cannot connect to T-Mobile's HSPA+ running on AWS. The update coming tomorrow is a carrier profile update which controls the phone's APN, LTE connectivity, Visual Voicemail, etc.
Starting on the 12th, the model A1428 iPhone 5 will be able to work on AWS for both HSPA+ and LTE. This is not a change which can be completed through a software update to existing devices.