Cellc Useless and *********** with account payments!!!

My mother is getting messages that her Cellc is in arrears. Which is IMPOSSIBLE as she has been making payments on time. This has been an ongoing struggle with Cellc. Proof of payments has been sent NUMEROUS times to try and get them to sort out the account. Still I get NO response through email, the account has still not been fixed and it is almost a year now!!!! They keep harrassing her that her account is in arrears, when we have been trying to get them to fix it on their side. This has been done through email - to which they do not respond - and over the phone - to which we are now tired of explaining and we will not be taking any further calls from Cellc as they are too *********** to assist. We want to deal through email so that we have proof of all correspondence done. This is really ridiculous and highly unprofessional!!!! Clearly the people in the accounts department do not know how to do their jobs correctly!!!