INSANE BILL DUE TO UNAUTHORISED SERVICE

Upon an upgrate in May i was not told my number which i haven't used for years will go from top-up to open line. My understanding was that i will only get data and an iPhone. I requested nothing on the old number. As i checked on my Vodacom app i noticed that there is like 1000megs on the number which i requested my partner to use since it has like 1GB not an open line. The bill that i got for this month was insane and i have not authorised for the number to be changed to open line. Because the person who was using the number couldnt check balances due to the number being an open line, she continued using it due to lack of knowledge since were never told it has now changed. I had to pay like 1800 that i did not budget for. Now i am being told it will not be possible to get a refund due to the usage