These problems are quiet usual in bsnl.. But to be safe on your side..
1) be sure that you dont have parallel connections before the splitter
2) be sure that all the connections from the phone box in front of your house to your system is lose connection free.. i mean, the two wires MAY touch each other or be exposed to outer climate. Sometimes rain can damp your speed. if the two phone wires ever touch each other/ spark, you get disconnected
3)if you are using torrents and getting low speeds, configure port forwarding.
4)and most of all if you get disconnection on a call, check weather the splitter is working properly as it fileter all the remaining voice frequencies to the router.
Hope it helps.. but its BSNL... so it mostly wont

.. Pray for the bsnl gusy to get a good start..
And i think you should post bsnl quries in the sticky only right? better verify the same