Vodafone Billing - Inappropriate conversion of prepaid number to postpaid

I visited Vodafone store to check the tax and service related to my postpaid number **********, the agent available informed me about the charges and gave me an offer to convert my prepaid number ********** to postpaid( under a family plan) informing me to get a single bill every month, I disagreed to it informing that my usage to prepaid is less. After several confirmation about the family plan, the agent assured me that I'll be getting single bill, to my surprise every month Vodafone is sending me * bills for my numbers asking me to pay them with a legal notice. I spoke to senior management they informed they'll take care but it is not being take care yet and I am again asked to pay bills. This is not expected out of Vodafone to a customer who is associated with service since more than ** years. I need an explanation and conversion of my postpaid number ********** to prepaid.