MTN billing for cancelled SIM

After countless calls to MTN they continue billing me for a SIM that was never activated. The contract was cancelled by the store because it was 'stuck in the system', however, MTN continue to bill me.

MTN departments are utterly useless. Customer service say I must speak to Retentions. Retentions say I must speak to Billing. Billing say I must speak to the store. The store confirms cancellation and around the circle we go again.

Nobody takes responsibility. Their systems do not integrate and it is simply impossible to get anyone to help.

Please avoid MTN if you can possibly help it.