unlocked iphone 4s reads sim not valid

hi guys,i am new to this iphone unlocking stuff.i would appreciate it if someone helped me out.i bought a iphone 4s from this guy which he said it was unlocked.when i went to pick up the phone i checked the iphone out and he had tmobile service on it.i had bought a att micro sim card and a tmobile sim card which both of them was prepaid.since i knew the phone worked on tmobile because he already had service with it i put in the att sim card and it worked with both.i even hooked it up to itunes and it recognized it as a iphone 4s.but when i got home i tried putting in the tmobile sim card it says sim not valid.i thought the sim was damaged and got another one from tmobile it still says invalid.does anyone have any suggetions.THANKS!!! it gets signal on it with up to 4 bars when tmobile sim card is in.it is version 5.0.1 (9a405) and the modem firmware is 1.0.13.model number of the iphone 4s is MC924LL.Thanks again.

 

sim not valid

how can it be locked by the network if it factory unlocked?what can i do if it is locked by the network.will they ask for receipt or ask where i bought it? when i brought it home it had the att sim card in it and i sync it with i tunes.you think that caused the problem?thanks