Contract over so not getting airtime but billed

We had a contract with Cell C and could upgrade in December 2014, with the contract expiring March 2015. However, money was still deducted from our account until 1/7/2015 even though my daughter transferred the number in question to her name. We did not receive any airtime in this time, but had to pay for it. Someone called us in last week and said if we don't pay outstanding amount, the account will be give to their lawyers, and when I tried to explain and get answers as to what was happening, the phone was rudely put down in my ear. I would like to ask someone to please get back to us as to why it is fair to deduct money from us, whilst not giving us anything in return. The phone number is not even on our name anymore, and still no one can explain. I have really lost my faith in Cell C and as a loyal customer for years, I am questioning the service and professionalism we have received as of late.