I am a long time T-Mobile customer and I am no longer under a contract with them. I am going to purchase a new, unlocked HTC Desire without a contract, but I am trying to find out if this phone (GSM850/900/1800/1900 & UMTS850/1900) is able to make use of T-Mobile's 3G. Anyone have a definitive answer? Thanks in advance for any help.
No. UMTS 1700mhz is the 3G frequency used by T-Mobile. The Desire is only compatible with AT&T 3G. The normal GSM frequencies (and EDGE data speeds) are compatible with T-Mobile (2G).
Mike is correct about the Desire. The only high power android smartphones that use T-Mobile's 3G are the Nexus One & the upcoming Samsung Vibrant (July 21st).
You're welcome. As a rule of thumb when foraying into the purchase of an unlocked phone, just make sure you have the proper bands for American carriers: AT&T - GSM (850/1900), 3G UMTS (850/1900) T-Mobile - GSM (850/1900 - though primarily 1900), UMTS (1700) Quad band phones are always compatible for regular phone service. Just keep in mind there are still a lot of Euro tri-band phones made (900/1800/1900), stay away from these as they lack (850), which is needed for AT&T. You can get away without the 850mhz band with T-Mobile, but if you travel around certain parts of the US, you'll lose service where they have 850mhz roaming agreements. Any other listed frequency bands are likely specific for Europe or Asia and the like.