From

GSM phones need the appropriate sim card for the GSM provider in the area where the phone will be used. Sim cards can be identified by a carrier designation code on the sim card itself. TFSIMC4 or TF64SIMC4 designate AT&T, TFSIMT5 designate T-Mobile. If you want to change the phone number of a GSM phone you will need a new sim card in order to do so.

So If you lookup your sim card code, it will tell you which network your phone is operating on. I got no way of knowing if that would be of importance.

Edit: except to say that ATT works much better than t-mobile in my home area.