Calling me for payment- but they owe me money! Payment allocations

For more than a year I have been making EFT payments on my contract because since the beginning of the contract in 2021 they have cut my number 3 times due to payment issues (not allocated on their side) Now for the past 2 weeks, they keep sms'ing and calling me for payments of May 2022 account. In actual fact, they have 2 payments unallocated which amounts to over R1000 putting me account into credit- they actually owe me money. 2 case numbers, 7 calls ranging from 13-24min each, 4 escalations, emails to collections depts (thrice), customer service (twice) and 3 separate agents directly. Cannot fathom that even though my bank beneficiary has remained the same with payment mailed directly to them from bank, and again from myself- somehow I am stuck with the problem. Escalations take 24-48hrs as per calls, 7 days has gone by. Collections take 24hrs as per email, 5 days still nothing. Severely disappointed, been a Cell C client for 20 years but this is ridiculous!