Changed my Plan without asking

I was using Flash 256 KBps plan, last month i recieved a mail from Tata Indicom that there is a special plan for you (infinty 500 KBPs), i replied to that mail asking more about plan . Few days back when i recieved my last bill, i shocked to know that they hav changed my plan to Infinity 500 KBps without even asking me . Tried calling customer care but as usual they are dumb, they refused to register my complaint & even i written mail to them but no reply till yet.Any1 can tell me to whom i can contact to degrade my plan to Flash 256 KBps ??