CELLC handed over my account that was 9 days in arrears without my knowledge

I had a contract with CELLC which expired on the 12th of August and I was due to pay my account end of August. I have failed to pay the account on the due date, due to financial constraints and arranged to pay on the 15th of September. On the 9th of September I received a call from some collection company and I was shocked to know that my account was handed over to lawyers by then my account was only 9 days in arrears. I was never contacted nor received a message from CELLC notifying me that my account is in arrears and will be handed over. I paid my account on the 23rd of September and on the 26th I called their contact center to check if the payment is reflecting and was told that the payment is not reflecting and I should send the proof of payment, I did that as instructed. I called on the 30th September to check if payment was received and it came as a shock that the amount paid hasn't been allocated to my bill/ debt . I am really frustrated and disgusted how CELLC quickly handed over my account that was 9 days in arrears without notifying me. Rectify the matter as I won't be liable for any interest incurred for this account. My bill was R249 and paid R270 therefore I will not pay any extra charges for this account. The time you wanted me to upgrade, CELLC will call almost everyday but failed to let me know that my account is in arrears. I'm guessing I'm treated this way because I refused to upgrade my contract with service provider.

Kindly rectify this ASAP and this is one of reasons I refused to renew my contract with these people, their service sucks and don't value their clients.