Bad Administration

I have settled my account with Vodacom as per their settlement letter which indicated the amount which is outstanding. I received the letter on the 23rd of March and paid on the 24th of March and on the 14th of April I received a statement and invoice informing that I have to pay R49.99. When I call I was transferred from one department to another like no one knew what was happening with my account. They informed me that my line has been deleted but question is why am I still paying for a line that doesn't exist.