No Service on Unlocked iPhone?

I purchased an iPhone that's AT&T, paid to unlock it, and got a new Tmobile micro Sim card.
I've been trying to get service on the phone but i've been getting nothing. Followed the steps from 2 customer service representatives from Tmobile, spoke to the guy who unlocked my phone too. And still nothing.

is it possible that the previous owner of the phone had disabled the phone from being used?

He could have sold you a phone with a bad "esn" meaning maybe your phone is blacklisted. Ask t-mobile to check the esn.

Have you checked the IMEI of the phone if it's blacklisted or not? Check with T-Mobile about it. If it clears, then the phone might not be unlocked yet. Have you restored the phone on iTunes after the unlock? If not, then do it and your phone will be unlocked.

No the previous owner has no authority to do that on such terms. You may talk to the retailer or say Tmobile itself.

Have you plugged it in iTunes first? Post unlocking, you need to sync it once to download the carrier data, and resync everytime you factory reset. If not, check with AT&T. You're on a GSM network so there are no such thing as ESN.

What do you mean you spoke with the guy that unlocked it? There's no "guy" the only way a phone can be unlocked is if the carrier that the phone was purchased from and the original owner of the device has got to be the person that asks. If you went to one of those businesses that claim to be able to unlock phones you wasted your money… But, I don't know what your whole situation is so just call us at 1800MyApple and ask to speak to iOS, When they transfer you, there will be an advisor on the line, Tell the advisor that you wanted to check and see if the phone was actually unlocked. We're the only ones that can check on that and as long as you have a new sim card that is activated you can get someone to help you get it all set up… I