Top up line billed as open line

on my August billing I discovered that one of the 5 lines I have with mtn was being incorrectly billed as an open line. this was meant to be a 2gig top up line. upon investigation I found that this has intermittently been happening over the previous 10 months.<br> <br> on logging the query I was advised that mtn would respond within 10 days.<br> <br> I visited 2 different mtn outlets quering the line in question and they confirmed that it was a 2gig top up line.<br> the 808 customer contact also confirmed this.<br> <br> today when I called in, I was advised that mtn needs 21 days to respond and that my line is now listed as open.<br> <br> for a line that was meant to cost me R120. IN ONE MONTH INCURED CHARGES IN EXCESS OF R3800