Some methods which work for me all the time.
For line faults
Call 1504 (option 1) and get your complaint number. Don't get frustrated with them. They're just there to forward your complaint. SNR Ratios and Line Attenuation, KBps or kbps mean nothing to them.
Dial 2xxx2198
xxx is YOUR exchange prefix.
MTNL Customer Care
Or else you can register it online here
MTNL MUMBAI :: COMPLAINT ONLINE
Make a note of your complaint number. Can't speak for all of MTNL, but the exchange in Borivli have been very quick to respond each time.
For Broadband complaints
1504 (option 3) for online help with a tech.
Again, your technical jargon or line speeds you see on your modem interface are meaningless to them.
Doesnt matter which Operating System, browser, or torrent client you use. All that they want to know is if your modem lights blink like they should, and if you are able to browse.
They will want you to do a speed test on this page. So giving them figures from other speed test websites is pointless.
http://202.159.230.232:8888/speed/index.jsp
My line speed had dropped from 2048kbps to 256kbps recently. Got it fixed by an online tech. However, my connection kept dropping. Got a linesman to replace some rusty cables at the telephone junction box. Everything is back to normal now.