screen freeze when making or receiving calls read description plz

Hi everyone

Well my question is i got an iphone 4 and its been working fine then suddenly it stop making or receiving calls everytime i make a call or someone call me the screen freeze. you can see the person calling you but i cant answer it or when i make a call i can dial the number but then it just stay on the screen as if its calling the person and i do get the missed call after rebooting the phone and i have restored the phone about 3 times already but still no luck so just wanted to know if there any parts i can change in the phone to fix it as i know its not a software problem and there is an app call viber and i can use the app to make and receive calls via wifi.

By the way my warranty is expired so i dont want to take it back to apple store.

Many Thanks in Advance

Raid