Cant get straight talk to work on my iphone 3gs?

I recently bought a SIM card for my iphone 3gs and i did everything i told me to do, i put in all the info and it still didnt work and when i called straight talk (twice) they told me that the t mobile sim card wont work on an at&t phone but it says on the card that is works for t mobile at&t and other unlocked or gsm smart phones so idk what the heck to do, if anyone has had this problem let me know so i can fix the stupid thing cause im going off to school in a few weeks and need a dang phone, and it was money wasted if not so somebody gime a hand here!

The first thing to realize here is that the customer service reps for Straight Talk have no training or actual experience with how to work on their own products, much less any phones that were brought over as part of the BYOD SIM card plans.

T-Mobile SIM cards most definitely work with AT&T products as they both use the same frequencies to transmit over 2G and 3G (part of why they can roam on each others towers).

Is the SIM card from a different Straight Talk phone? If so, that will not work as all of the GSM Straight Talk phones aside from the Nokia E-5 and E71 have their SIM cards linked to the phones they were sold with as a means to control what their customers can do with their phones.

If the SIM card is one of those that shipped from Straight Talk's BYOD SIM card plan, everything should work, unless Straight Talk managed to screw things up on their end, which is not uncommon.

Does the phone not work at all, or is only part of it working, such as talk and SMS working, but not data and MMS?

Also, does the phone have a clean ESN (IMEI)? If not, then that would explain the entire problem, if not, then the problem lies with Straight Talk.

I will check back in a few days to see what the response is to my questions, and I will try my damndest to help you get a solution found.