faulty white iphone 4s ?

White 16gb iphone 4s The reason this phone is listed a faulty and so cheap is for some reason it has developed a problem of not being able to make or recieve calls, the first thing that came to my head was it was bared but thats not the case as i can still use the 3g internet from the provider so the sim is still working, i have had the phone for nearlly 4 weeks and never has had any other problems before now. im selling as im getting a new upgrade, so dint need two phones and havent got money to fix this, may be a cheap fix for some one who knows what there doing. also the power button is a little sticky but works fine every time, being sold as seen so no returns as i have been honest in the listing that it is faulty brian on 07810432061