Being billed for a number that I cancelled

Last year I opted not to renew one of my cellphone contract with MTN and up to date I being charged for this contract even though I have been calling their call centre to rectify this. I have decided to stop paying any account and last week receive a letter of demand to make payment by the 26th of March 2016. I then send an email again to inform them to rectify my account first. Today my other active cellphone has been suspended. Tried to call their call centre this morning and was told their system is down hence can't help I must call later. I am fed up with MTN service to say the least.