They are unable to allocate my payment

i owed old mutual staff medical account in 2014 i got a call from credit worx (Nimble Group) to say the account has been allocated to them. in March 2015 i paid the full amount via Cellphone banking. I got a call saying they haven't received payment and that i should send them my bank statement reflecting the amount, i did that and still nothing was done and now they want proper proof. i got a message today that i now owe double the initial amount. i have been calling since March and when they have to put me through i get disconnected have sent emails they respond when they feel like responding. how do i settle the account when no one is there to assist???