Misguided by Vodacom

In January I received a call from Vodacom consultant and told that because I am a loyal customer my data will be increased by 50%, and I then ask if that won't affect my payment to them and she said no I will still pay the same amount I was paying but to my surprise I was billed almost twice the amount I use to pay, I called and tell them to cancel the 50% added, which they did but I still owe them February amount