Iphone 4 locked to original sim? T-mobile?

Brough a iphone 4 from a well known auction site, came with box and everything was told it was a old contract and they got a new phone now. So i brought a micro t mobile sim from EE store and all that happeneds is the signal will go high then drops completly saying EE or T-mobile then it would say no sim. (as the seller lived near me i contacted them went rond there and they showed me the phone working fine with there contract sim card in the iphone) So am i lead to believe that the iphone 4 will only work on that original contact sim card? And if so how do i go about unlocking that as i now have a t-mobile sim with credit i need to use.

YEs, your iphone is locked to a certain carrier, and would only work with that sim card. In order to make it work with any carrier worldwide, you need to get it factory unlocked. If you are interessted, send me an email at stefan_zxcvb@live.com and i will explain everything.