Battery problem

Please be informed that I buyed the I - Phone 4 S from Vodafone Egypt and there was a problem in the battery it couln't be charged. I returned the phone back and they said they will fix the problem but after 3 weeks trying to solve the problem they returned the phone back to me with the same problem. My Mobile no. is : +201000122290, please solve me the problem the phone is still in the guarantee and this is a technical problem so it's my tright to get a new phone.