If i bought a AT&T iPhone 4, can i pop my tmobile sim card in and it will work?

I purchased an AT&T iPhone off ebay and i was wondering, when i receive the phone can i just pop in my tmobile sim card and it will work or do i have to go into an AT&T store?

I believe your AT&T phone need to be unlock to be used on tmobile.

No. They don't work with T-mobile sim cards. You have to unlock first if you want to use it with T-mobile sim card. Information that i know it's hard to unlock devices on AT&T. But you can go to some mobile services and ask them if they could to unlcok.

Your phone needs to be unlocked first. You can try though. T-Mobile has micro sim cards for like.99 if you buy from their website. When I bought my unlocked iPhone, the rep at the T-Mobile store just gave me the micro sim for free, I just had to pay for the prepaid minutes.

No. The phone would need to be unlocked in order for other carrier SIMs to be able to work in it.AT&T could help, but I think the person making the inquiry about getting it unlocked would have to be the original owner. Think, but at the same time doubt.