Clearly whoever you spoke to has NO IDEA what he is talking about.
Any standard ADSL modem will give you any speed up to 8mbit/s (assuming the line is in excellent condition and you are close to the exchange).
Any modem sold within the last couple of years, however, is likely to be ADSL2+ capable and therefore capable of up to 24mbit/s (again, assuming the line is in excellent condition and you are close to the exchange and the exchange is equipped for ADSL2+).
The modem in question, it seems, is this one: Powered by Google Docs which is actually capable of ADSL2+ and therefore would handle the speeds up to 24mbit/s, unless the modem was given to you by BSNL and they intentionally crippled it (in which case, all that is needed is a firmware upgrade).
Call the BSNL guy back and tell him he is a moron.