My iPhone 4s wont connect to the server data service..

i have the same problem.. have a new 4S (unlocked from factory), came to Spain, bought a pre-paid sim card, it works perfectly for calling an dwith wifi but wont connect to the 3G (it reads 3G in the phone..)

Ive called the cellphone company and they've sent all the things they send to make it right.. nothing.. called the expert in iphones from the celphone company and she said to reset my phone at home, did that.. nothing..

i have no idea what to do next.. i tried another sim and it doesnt work, while my sim card works fine in other iphones..

thanks for any help!