Sending a reply from your mobile phone
Replies are open to anyone whose account isn’t protected, so you can reply to any Twitter user from your phone. This is great if you want to send a witty retort while you’re out and about.
Here’s how it’s done:
1. On your mobile phone, access the SMS feature and start a new message.
2. Enter the Twitter phone number for your locale.
3. Type @ followed by the Twitter username of the person you want to reply to. Be sure to add at least
one space after the username.
4. Type the rest of your message.
5. Send the message. Your phone hands the message off to Twitter, and it then appears on your timeline, usually within a few seconds.
Reference : wiley.com