Non payment of PMB related tests

I have been querying an account for PMB related blood tests for months! The account has now been handed over and when I contact the call center I am continuously told that the consultant is still busy with it and they will ask her to give me a call. No one ever calls! The account is from November last year and no one can answer as to why it is still not paid. Can someone contact me as to when it will be paid?!! I now have debt collectors calling me.