1% VAT increase changes my contract agreement

Vodacom is continuously debiting my account with different amounts , every month my contract increase when u call their call Centre they will tell u its 1% VAT increase, How can a contract of R299.00 per month increase to R371.00 after year of having a phone. the phone is 1 year old and I don't understand how its value appreciates. how can 1% of VAT increase be equivalent to 71.00 . This is really bad service because they don't even contact you and let you know.