Being billed for migrated sim card.

MTN is useless. The most *********** service provider in the country. <br> I migrated one of my contracts, as per MTN policy, that was due to expire. I gave 30 days notice for the cancellation. Only for them to migrate the wrong number. Then when called to follow up they migrated the correct number, however I am still being billed for the number that was switched to Prepaid in June/July. However the did not give any airtime or sms bundles. My question to them what are you billing me for if the number is cancelled and no service is received? I wish to speak to a the Head of Retentions or the Head of the Call Centre.