Would an unlocked 3G t-mobile phone work on AT&T 3G?

[bob]

My understanding is at&t uses 1900 Mhz for 3G and tmobile uses both 1700Mhz
and 1900Mhz (one to send one to receive).

Many gsm phones by default supports 1900Mhz but not 1700Mhz.

Some phones sold by t-mobile are custom version to support tmobile 3G. E.g.
samsung galaxy S II.

Since a tmobile 3G phone needs to support both 1700 and 1900 Mhz, does that
mean an unlocked 3G tmobile phone would always work on at&t 3G network?

[tlvp]

Post by bob
My understanding is at&t uses 1900 Mhz for 3G and tmobile uses both 1700Mhz
and 1900Mhz (one to send one to receive).
Many gsm phones by default supports 1900Mhz but not 1700Mhz.
Some phones sold by t-mobile are custom version to support tmobile 3G. E.g.
samsung galaxy S II.
Since a tmobile 3G phone needs to support both 1700 and 1900 Mhz, does that
mean an unlocked 3G tmobile phone would always work on at&t 3G network?

Alas, at&t may also use the 850 Mhz band in some regions. There (as also in
parts of Ecuador) even a tri-band T-Mo phone is dead in the water, as 850
is not one of the 3 frequencies (900, 1800, and 1900 MHz) a T-Mo tri-band
supports. A quad-band T-Mo, phone, otoh, will be OK there (850 Mhz is the
"quad-th" band :-) ). Certainly a quad-band T-Mo phone will be good on at&t
data roaming up through EDGE data speeds as well, but HS*PA will fail, as
T-Mo handsets expect to use the 1700 MHz band whereas at&t towers require
handsets that expect to be using 1900 :-) .

HTH. Cheers, -- tlvp