Cell C - ********** Debt Collection Company working with Cell C *****s money

I am trying to do an upgrade on my phone. I have been with Cell C for 10+ years. I had an overdue account transferred to a collection agency CSS - Credit Solution Services. I paid this up with CSS and received the settlement letter in March 2022. To date CSS has never notified Cell C that this has been settled. Cell C will also not accept the settlement letter to clear the overdue account, which has now been written off, and giving me red credit status. I have spoken to over 10 agents, mailed customer services, credit department, billing department etc. Some try and help, but never call back or send an e-mail, and others flat out refuse to help, and pass me on to the next department. This is the last try to sort this out. After this im leaving, and telling all my friends and family to do the same.