For people who are off net and would like to recieve @replies from people(on Twitter) as SMS ,so they respond back, Twe2 was one such service. But it is not functional now. You can read more here.
So an alternative would be to create one for yourself, all by yourself using Google SMS channel.
Create an account in Google SMS Channels or you can usee your Gmail ID(http://labs.google.co.in/smschannels/browse),
Select "Try Now" from Create you own channel on the right side.
Next page provide a name, description,category,location as you please.
In the Source select "RSS/Atom feed" and provide the following link [ search.twitter.com/search.atom?q=%40yourusername ]
Replace yourusername with your twitter handle like mine - Sumeet.
In "content" Select Body.
In "Allow publishing by" select Only me
In "Who can subscribe" select By invitation only, Since you would like the replies to be private.
And lastly agree to TOS of google and press Create channel button.
Since this was just an experiment I tried there were two shortcomings.
Firstly, I couldn't get the senders name(big problem).
Secondly, the messages arrive almost after an half an hour.(Only Airtel showed this behaviour)
Until next time.