Avoid the WAG 120N, I bought one a few weeks back and I'm not happy with it at all.
Very Low Wi-Fi range
Sometimes does not connect to DSL on boot
Does not display SNR / Attenuation stats so you can't see line quality.
On the same line the Beetel modem provided by Airtel was working fine.
I would advise the same as thatsashok, use BSNL's modem otherwise they might blame all issues on your router.