3G phone for both AT&T and T-mobile?

[bob]

I believe for 3G to work, the phone has to support UMTS 1900 Mhz for AT&T,
and UMTS 1700/2100 for t-mobile, right?

Which gsm phones, when unlocked, would work on both AT&T and t-mobile in the
usa?

[tlvp]

Post by bob
I believe for 3G to work, the phone has to support UMTS 1900 Mhz for AT&T,
and UMTS 1700/2100 for t-mobile, right?
Which gsm phones, when unlocked, would work on both AT&T and t-mobile in the
usa?

Pretty much all of them with at least the 1900 GSM radio. (Only the
European/Asian 900/1800 radios are useless in the USA.) I've used an
unlocked Cingular Moto SLVR L2 on T-Mobile with no difficulty, and I'm
currently using an unlocked Cingular LG cu-400 with no difficulty, either.
Voice, voicemail, SMS, MMS, even Opera Mini, all do -- or did -- just fine
on both handsets with T-Mo.

They just can't exploit T-Mo's higher speed 1700 services -- but only one
of them would have been able to exploit Cingular's higher speed 1900
services, anyway, so no great loss :-) .

You might want to be sure to have an 850 radio as well, as there are places
where at&t has only 850 services, and no 1900 ones. But T-Mo GSM is 1900
only, AFAIK.

HTH. And pardon my delay in responding. Cheers, -- tlvp