Credit balance statement

I took a contract with Vodacom in 2009 and it was handed over to Jacob Burnard.I have been paying until last 2013. Early this year they contacted me again asking me to pay then i requested a detailed statement that was sent. the statement has a credit balance of R2000.00 and they still want me to pay the account that has a credit balance but she ask me to proof with a paid up letter. i have a statement from them with a credit balance. this people are reaping people off.people must know that they are collecting even if they account is paid in full. i want a paid up letter and refund that is due to me