HEY I GOT My iphone today, which is supposed to be fully unlocked, it came with an ATT sim which obviosuly had no service, but when i put my SIM, which works with a mexican carrier called TELCEL with sealthsim v3 , the phone is not able to access WI FI or whenever i try to dial a telephone number, the screen blacks out and sends me to the main menu, also when i try to write a SMS, the phone sends me back to the main menu. this all happens when my mexican sim is in, but whenever i put the ATT one, the phone works fine but obviously without service, wierd thing is that when my mexican sim is in, i do receive phone calls any time i dial, but when i want to check this numbers, i can not access, as the phone locks itself,

Check the Turbosim etc... section under the iphone category. I am sure I've seen an answer in here at some stage.

Looks like the symptoms of being in an "unsupported country". You might want to try to install iWorld, run it and set the phone for your country.
I hope this solves it for you.
Cheers, and a Happy New Year!
mousyak