yea u r rite.. u need a dail-up modem.. a gud D-Link modem will cost u about 350. it shud be installed in a pci slot inside ur computer.. it will hav 2 phone jack inserting slots.. insert ur main phone line in one of them and connect the wire (wich comes wit the modem) from ur modem to ur phone.
You might have 100 hours free but with normal dial-up its not the only bill . You will be billed as per mtnl rates like 1 rupee/min or whatever for he time you use internet over phone . Get a cheap pci modem too.
It's not Re 1/min. But you will have to pay for the telephone usage charge in addition to the 100 hours you paid to sify. The telephone charge in different in peak and off-peak hours. I think its 120 seconds for peak and 180 seconds for off-peak hours.