Vodafone — Problem with free talk time

My name is Surajit Chattopadhyay and I stay at 1/19 Dover Place, Kolkata 700019, West Bengal.
On July 29th of 2008 I recharged my vodafone (no. [protected]) with Rs 99/-+Rs 148/-+Rs 51/- in which Rs 148/- is for 750 minutes free talktime from vodafone to vodafone (in a month). Suddenly and most surprisingly I found on 14th of August, 2008 that my free talk time has reduced to on 55 minutes only which is absolutely impossible because till that date I had made only a few calls to other Vodafone numbers.It has created a serious problem to me. Such an irresponsible service is not expected from a company like Vodafone. Nowadays it is found that in most of the times they do not show the remaining talktime after making a Vodafone call. I want an immediate solution to my problem.