Hello Friend. I experienced the exact problems you are currently facing in the very first year i got my BSNL DataOne Broadband connection.
Presently, i do not face the problem you have mentioned in your initial post. Perhaps, it is because our local telephone exchange shifted all the lines (phase by phase) to a "new" exchange. This is what our local JE told me over a telephonic conversation.
Coming to your problem, i would suggest that you inspect your telephone line personally or better, request your local lineman to do it for you. Make sure there are no joints at various lengths of the wire. If the need be, request your telephone lineman or a higher authority to change your existing telephone wire with a new one.
Also, check the ADSL statistics on your Router page. Type 192.168.1.1 in the address bar of your web browser. The User Name & Password for the router page is
admin.
In your Router page, look out for SNR Margin (dB) & Attenuation (dB).
This could help & serve as a reference -
Have patience & wait for the apt replies of the more experienced techs here. I am confident they will certainly assist & guide you in troubleshooting your problem.