DUMP CONTRACT

So i had a contract with Cell C and it was fine for some time then they decided block the number,i couldn't do anything with it..I called then they told me i had more than 1 number with them and it was overdue. I advised that i only had one contract and they said they would cancel the one i knew nothing about ..They did and i carried on using the phone till my former employer called me and advised that id been handed over. I called them asked them to cancel my contract and give me the amount to pay...They did so then gave the amount to pay and i paid. I figured it was all over,then this morning i got an sms saying i've been handed over to nudebt and should pay 359.11...Thinking okay i'll call and remind them that i cancelled. Someone then called and said the amount is not +500.. Im furious as it is and your call center people say nothing ...PLEASE ASSIST ....WHILST IM STILL SAYING PLEASE