Your phone doesn't control which voice network you connect to, but rather you SIM card controls it. However, your phone is in charge of assigned a name to that network and dispaying that name to you. Import phones don't always know the correct name to show here in the US.
Are you by any chance in California? If so, due to the tower and merger integration of Cingular and AT&T and Cingular's gradual handover of all towers in CA (and a fewe other markets) to T-Mobile, your phone might be calling it a Cingular tower.
If you're not in CA, NV or NY where this integration and handover is gradually taking place (and tower names are in flux) then you could go into phone settings and set the phone to use manual rather than automatic connection to your carrier. However, the phone generally follows the SIM's roaming list and lack of signal for your home carrier which means you might not have a strong signal on T-Mo.
-------------------- Lisa Gade Editor in Chief, MobileTechReview