Billing on cellphone

Debit order returned, contacted the service provider in December 2014 to settle the account on multiple occasions. They responded saying that my account is up to date, at a later stage I received a lawyers / attorneys letter stating that I must confirm settlement of the account, which was done. Now I receive an account of R 6 689.55 four months later without my knowledge or confirmation thereof. The contract is now due for an upgrade which is being refused until I pay the outstanding balance.