iPhone 4s won't call/txt anything. Help

hi guys,
im a newvie to all this but found myself stuck, i was given a brand new iphone 4s from my brother who got a new contract but only wanted the sim. i was useing the phone happily for a few weeks, now i cant make /recieve calls or txts.
the phone only functions when on wi-fi, iv tried using a differnent sim i know works but have same issues, could anyone please advise, thanks
C