Account Overcharged

I had two contracts with vodacom which were not taken on the same date. Last year I experienced difficulties of paying my account with them and the other contract was ending december last year, while the other one was due this year May. I received a letter from Vodacom saying that I'm owing R10 000.00 which was such a surprise to me cause I never used the sim cards. When I checked my transunion report it reflect R3400.00. So I need to know how come does Munnik Basson and Dagama and Associate are saying I'm owing them R11 500.00.