3G network does not work

Hi,

i have a iPhone 3GS and when i try to turn on the 3G network, on the top of the screen it's shown "No service". I can't make any call, text or internet using 3G, even if the area supports it, but if i turn off it and use the edge/gprs network everything works perfectly. I also tried to restore all settings, network settings and set the apn values but it still doesn't work. What could it be? I've finished my Apple's guarantee period so i hope it isn't a hardware problem 😮

Thanks