was billed about R1545

I have two cell phone contract numbers with MTN, from 2013 October, I was due for upgrade in November this year I decided not to upgrade the other cellphone both my cell phones equate to about R380. Now at the beginning of December R1500 was debited by MTN I then asked I was told I got a third cellphone number I have upgraded with and it's an open line, I had never upgraded with a third number the number, they showed me it my first time to see it no body knows which mtn store was the upgrade done. I still say I know nothing about this third contract it's the most insane contract ever it's so expensive no ways I can afford that