Mr. Price mobile making me pay for an expired contract with them that I.Mind you i did not cancel the contract..i let it run to expiry

2017 i took a telephonic contract with Mr.price for a mobicel cellphone..the phone died in about 4 months..i stopped using thier sim card and allowed the contract to debit until date of expiry..one of thier consultants called me lastyeat and reminded me that my contract was to expire and asked if i would like to renew my contract and get a new phone.I told the lady that i would like to discontinue my contract and not renew it..she said ok and told me my last payment date..i waited till that day and allowed them to debit..All of a sudden this year thier lawers call me and tell me to pay R1500 rands...I ask them what for and i dont get a reply..Iv explain this story to everyone of their consultants calling me.The last one told me to just pay because there is no way im getting out of it..but im getting explanations as to why i should pay this mysterious amount of money of which im alleged to be owing..Now they are threatening me with legal action.Any advice on how to handle this matter?