You seem to be very helpful on these boards. I have something weird and cannot explain. I bought two 'unlocked' SE phones. One is the T616 and the other T610. First one uses 850/1800/1900 and the second one 900/1800/1900 bands. I used the 850 one in England with a local SIM card from Virgin Mobile and it worked perfectly (even though UK is 900/1800 bands). When I turn the phone on here in US (with the UK SIM card in it, it says T-Mobile on the screen) even though originally the phone was from ATT.
Now, I also found out that the second one, with 900 band, was NOT unlocked, and the store is out of business. When I put in an ATT SIM card it does not recognize it. However when I put in the Virgin Mobile it works on T-Mobile frequency, as I am bale to turn the phone on.
How is that possible? I thought if it is locked it should only accept T-Mobile SIM cards and no other.
Can I assume T-Mobile and Virgin Mobile are somewhat 'sister' companies? I am traveling to Italy and I want to know if there is a possibility the 900 band one will work there just like the fact that it is accepting Virgin Mobile SIM cards?
Sorry for the length of this but would appreciate your input and expert opinion.
Thanks.

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