Contract finished debt handed over to 3 companies that don't communicate

I'm soo unhappy with Cell C, I would never advise anyone to take up a contract with them. At the end of my contract, I wasn't even told my contract is finished. I was just charged an outrageous amount without my knowledge....as if that was not enough, my account got handed over. I paid the account with HP attorneys around April, even today I am still getting calls from other debt collectors coz they sell their debt to more than 3 companies that don't even communicate. You pay with one and still get calls from other companies, having to always explain yourself that you paid and all only to get more calls the following day....someone please direct me to someone who can assist me. I'm tired of these calls even after I payed what I owed them.😡😡😡