My iphone wont let me call or text!

Okay so i just got an iphone 4s it was my dads old one (i got it when his contract ran out) Anyway as soon as i put my sim in its registering my sim is there and i have full signal and everything but it wont send any texts or let me call anyone but i can receive calls and texts, anyone got any idea whats wrong and how i can fix it?

What happened when you used someone else's phone to call your provider and ask them for help? I would just follow the advice of the customer service rep you have surely called by now.