Not Recognizing SIM Card- PLEASE HELP!!

[redsox_sweetie]

Ok, so yesterday I dropped my cell phone on the ground. I have dropped it numerous times in the past and never had a problem. After I dropped it, it still worked for a few hours. Then, in the middle of a call, it cut out. I looked at the screen and it said "SOS". After a few minutes, I took the battery out and re-set it. It worked for a few more minutes, and cut out again. This time it also said "Insert SIM Card". I tried re-setting it a few more times and no change. I went to a phone store and they tried putting an AT&T card in it, and it didnt work (Its an unlocked AT&T Cell phone, that Im using with T-Mobile). They put a pre-paid T-mobile card in and it worked. They then tried a new regular t-mobile card and it didnt work. I have no idea why the pre-paid worked and no other ones will. I am positive the SIM card is in correctly, and I have tried everything. I really dont want to have to buy a new phone. Please help, any ideas would be appreciated....

[312]

This actually is in the wrong forum, but...

1) Are you sure both SIMs were active
2) Are you sure the phone isn't a locked T-Mobile and you tried to insert an AT&T SIM?