CELL C COLLECTIONS REALY BAD

I have been a Cell C customer for 18 years and never missed 1 payment. I asked for a settlement figure in August 2021 which they gave me telephonically and then sent an sms. I paid the settlement figure in August, and they ported my number to Vodacom. Now they insist I owe them money. I mailed the customer services and collections numerous times. I do get a reference number but no reply. The collections department send me a notification threatening me with a final letter of demand!!!! I mailed them again 3 times and still no reaction from collections.