handed over without notification or letter

I had medical aid with discover for 3 months.then I was employed at Group Africa marketing,in May 2014 I was retrenched and canceled the medical aid and notified them I was retrenched,in August I received a letter that says I owe them R2880.00 because my savings got lapse when I visited the optometrist,i tried to call them on the number on the letter but did not receive any response,in 25/11 I paid 500 4/12 paid 300 and now in 15/12 I paid 1500.00(I sent them proof of payment,and then I received a call from some lawyers saying discovery has handed me over to them and I owe the 3400,i said to the lawyers im not going to pay the 3400 and I have been paying my account at the bank account number that discovery sent on the letter and im not going to pay into their account since discovery did not notify me of any hand overs,so now this lawyers are threatening me that they will black list me to credit bureau if I don't pay the 3400.00 to them