Duh me.. if I have a bought a t-mobile phone (used) from someone and i

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have t-mobile do i have to get it unlo? do i have to get it unlocked? or is it already unlocked if they had an account...please let me know? I am clueless..and if it is not unlocked I assume I have to get it unlocked for texting/phone use. And how much do I have to pay someone to unlock it..i do not want to get in trouble if someone is unlocking for me...i want to make sure it is legal for someone to unlock phone..i see ads on forums for this...thanks..