Overcharging

I upgraded my sons cell phone in 2014 from a blackberry contract to a samsung contract. i have noticed that since then i was still charged by mtn for BIS although it wasnt on a BB contract anymore. i then emailed them, and obviously i had to sent a few reminders. i eventually got a curt response that BIS was de-activated. i then rep**** requesting credit on the 12+ months i was over charged and surprise surprise, no response from mtn, even though i sent reminders. mtn still refuses to give my phone back that was sent for repairs and i did not accept the quotation. i am being charged a \service \" fee, but they do not want to sent me a invoice with their banking details and VAT number so that i can make payment, their excuse is that they can only issue invoices AFTER payment is done, which to me is strange and suspicious. but for now, i just want a credit on my account for the money they over charged me with"