The Rezound is a Global Mode phone, it allows for CDMA and GSM networks with the appropriate SIM card on compatible GSM networks... That being said, you would be very disappointed with the Rezound on T-Mobile (or any other GSM carrier in the US) as the radios are not setup for the proper frequency and tuning for T-Mobile/AT&T, if you get the SIM it will work as long as you are on the latest firmware, but only for voice and SMS messaging... you will not get data (other than EDGE, which you might as well not have data, it is like 10-15k speeds IF it works at all) and other services like MMS messaging.
This has been discussed over and over, with T-Mobile, Straight Talk, Net10, AIO Wireless, etc... If you go on a AT&T based provider, you might get HSPA data, but it usually requires severe tweaking in the NV settings... not a procedure to be taken lightly as bricking is a possibility.
I went through this for two months, trying everything possible. I finally gave up and moved to a Nexus 4 with an AT&T Go Phone SIM and couldn't be happier... I would have preferred to continue using my Rezound, but it was just unusable on GSM service in the US. Note that if you go outside the US, the frequencies are different and the phone works acceptably with HSPA, and sometimes even HSPA+ data, in markets such as the UK and the EU.
Sorry to be the barrier of bad news, but it is what it is.