A cell phone having Band I
will work on USA T-mobile, if it also has the other bands.
I think you meant to say a phone having
only Band I will not work on
USA T-Mobile?
Just clarifying "to avoid confusion".

You can't make a blanket statement, saying
"a phone having Band I (2100) will not work on T-Mobile."
Two reasons that statement would be incorrect:
1) Phones
with Band I will definitely work on USA T-mobile -- if they are pentaband phones. Yes, they have Band I!
2) PLUS,
T-Mobile actually operates on Band 1 -- in Europe.

Yes,
"T-Mobile" operates on Band I. T-Mobile (and using the T-Mobile brand name) operates in 11 European countries and use Band I in some of those countries.
Sure, here in this forum, "USA" T-Mobile is normally assumed, but you are the one who introduced the topic of European Band I and specifically mentioned
T-Mobile.
So, if you insist on talking about European Band I and T-Mobile, you have to acknowlege T-Mobile DOES operate in that band in Europe and limit the discussion to
USA T-Mobile.
So, if we are talking about USA T-Mobile and phones with Band I capability...
MY phone has
Band I and
works just fine on USA T-Mobile 3G (HSPA+). 
Of course, it has Band I
and others. It's a penta-band phone.
(above)
A Galaxy Nexus with Band 1 capability on ST "T-Mobile" sim as well as official T-mobile sim
Penta-Band 3G for these 5 frequency "bands": 850/900/1700/1900/2100mhz.
The unlocked "international"
GSM Galaxy Nexus ($350 at Google Play) is a
pentaband phone and works on ALL those bands, thus is the safe choice for both ST "AT&T" and "T-Mobile" capability. On Straight Talk, the Galaxy Nexus will run on HSPA+ speeds on both ST "AT&T"
and ST "T-Moible" towers. Bonus is the fantastic specs!
This is the nice-looking pentaband
Meizu MX:
Some are easy to get ahold of, some are harder. But they all have Band I
and will run on USA T-Mobile. Bonus is if you ever travel to Europe or South America on vacation or business, your pentaband phone will still work with cheap local pre-paid sim cards!
Again, the Galaxy Nexus is the safe choice!
Once you get your pentaband phone, just choose the sim for the better of the two HPSA+ networks in your area. For me it was the ST "AT&T" sim, but when AT&T towers are not available, my phone will ALSO run on the T-Mobile towers at HSPA+ speeds. An AT&T branded phone will not do that, as they have no pentaband phones. If an AT&T branded phone on Straight Talk is able to access T-Mobile's towers, it will do so only at the very slow Edge speeds, as it does not have the proper chipsets for HSPA+.
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