Tata Indicom — Change of present call rates

Dear Sir,
I am using tata indicom connection (Will rovide the No if required) since last 1 year. I have taken Plan 275 which has std call rates @ Rs 0.50/-. Today on 14.07.2009, i have received one letter dtd 08.07.2009, which states the change of call rates to Rs 1.00/- after limited calls of 2000 minutes. At the time when i had taken connection there was no such limitations of calls thats why i had opted the same plan.
Is that legal to change the call rates in between untill i surrender the connection?
I request your necessary action regarding this.

Regards

Amit Kumar