over charging

since December we have problems cell c is over charging us we tried sorting this out then we spoke to a lady on whatsapp who acknowledge its her fault but can we please pay she will pay us back monthly she cannot afford much as she is a single mother and if her boss finds out she will be fired i do have her whatsapp number and i do have her voicenotes in the meantime they cut our line and threaten to hand us over we do have all the proof that we did pay every month this is unacceptable we are with cell c for 20 years now