Worst Service provider

Vodacom is the worst service provider out of all the Mobile network providers we have available. I've had a contract with Vodacom since 2012, in 2015 when I joined my previous employer, who was giving me cellphone allowance, my account was changed to a sponsored account. 3 Months ago I resigned from that employer and carried on paying Vodacom every month end. This month they have been harrasing me and sending me messages that I have not paid my account wherelse I did pay in full, infact I paid in advance. After calling them and sort out the issue as I had POP as well, then they said my account number had changed from KI*** which I had been using for years to I***2. Then I was told, I need to send the POP to *** and ask that my money be moved to the correct account. I did that, but up until today they have not moved or updated my account, it still show arears and will affect my credit profile as Im in a process of buying a house. How do you change a customers account number without letting them know?