Sucking money from innocent people.

I got call from a law firm stating that I owe body 1800 rands, what's I don't get is that I signed a 1 year contract with them of which I paid up in full on the second month after I joined. Earlier this year they made me pay 250 rand stating it was a maintenance fee of which I paid. Now they hand me over to lawyers stating that I owe them 1800? I refused to pay and advice them to open case against me or take me to court.