I lost my phone which had a BSNL prepaid connection. I called up customer care to find out the procedure for cancellation and subsequent replacement of SIM card. They told me that I would have to get a police complaint and a fax to a particular number stating my connection details for cancellation. I went and got the complaint and faxed the details to BSNL. I subsequently called up customer care who stated that their system was non-functional at the time and to call later. The next day when I called again, they stated that they had not received the fax (though I had got the fax tone before sending and had a transmission verification report). I resent the details only to hear again from them that they had not received the fax. During this time, my phone was still active and someone was utilizing whatever talktime I had.
Other providers would let you block the SIM over the phone and have a simple procedure (without Police complaint) to get a replacement.