Vodafone

I have been using vodafone service since last three years. Before one year the Company changed my tariff plan with out any informtion when I approched customer service center they missguide me and did not provide me correct solution.After that my monthly billing changed now recently company again changed my tariff plan and removed all previous facilities in my plan with out any informstion and send me huge amount bill.And forced me to pay the bill.Again I approched to the service center they did not provide me any resolution the cause of error besides forced me to pay bill.Several times I mailed to concern person but till yet did not get any resolution the company threaten me and send me legal notice.This is my request to you please suggest me what should I do.