switching contract/number to another party

Since April 2014 our business line was transferred to another company that bought our clientele. Both parties involved in the transaction has been to more than one MTN store to assist. At the Irene store it looked like we were getting someone who knows what he is doing and he instructed me to hand the sim over to the \new owner"and will sort all the necessary document with her. All legal required documents were submitted and all requests has been met. MTN just do not switch number to the other party and has been billing us for 18 months even we haven't signed a new contract with them. Numerous calls and emails to customer care and employees all just resulted in no help at all. The odd thing is that the party taking the phone number from us"