Log a complaint with BSNL about the issue. If they find no issues in the line, and tell u that it's a modem problem, then ask them for a test modem for checking if it's really a modem problem. They may loan a test modem to u for a few days.
Here are some steps to verify if its the modem problem yourself.
0-
Check for noise in the receiver
If you are getting noise in the receiver or immediate sounds while talking then its either a line or splitter problem. Connect the telephone directly to the main line, bypassing the splitter. if the problem continues, either your SNR is low or the parametres are low (get the BSNL BB technician to check your L5 and L6 levels. if L5 is below 400 you are bound to have problems. for L6 its below 200. Normal levels are 1016 and 1016).You may also want to check the roset box and line cable for carbon in this part.
If not,
1- Bypass the Splitter
Connect the modem directly to the line
2-Shift from PPPOE to Bridge mode.
If your modem connects to the internet automatically, then you need to change it to connect manually
3-Connect using a LAN cable.
Avoid using wireless data transmission.
4- Diagnose
While the modem is connected, ping google.com -t and log for 5 mins. if there are no packet drops then the line is working as intended. If there are then refer to point 0
ping 192.168.1.1 -t (the modems ip unless you have changed it). If there are no packet drops then the modem is transmitting the data well. if there is, the modem or lan card is faulty. (check accordingly with another pc or laptop)
5-Use another modem.
If everything else fails, you need to check with another modem. Ask the BB technician to test with his modem for 5 minutes. if no problem is shown through his modem, you need to change yours.
NOTE-: A modems life expectancy is maximum 2 years. it should be changed after that.
Change your splitter and phone cables every six months.