You must sign up for Twitter from your mobile phone or web.
To register via Web Channel: After opening your Twitter account, follow the Settings and Mobile options and register your phone number.
To register via Mobile Phone: Write “START” and send it to 2444. In messages from Twitter, you will be asked for your username and password respectively. Do not proceed to the next step without waiting for the confirmation message. Finally, you will be asked to write “OK” and send it to 2444. After this stage, you will receive the message that your transaction has been completed successfully.
To send a tweet: Send your “tweets” via SMS to 2444
To receive the tweets of the people you follow via SMS to their mobile phones: When you hover over the name of the user you want to follow on the Twitter web page, simply click on the phone icon in the lower left corner. To do the same thing from your mobile phone, just type “FOLLOW username” (username of the person you want to follow) and send an SMS to 2444.
You can visit the Twitter Help Center page to see what commands and what you can do with SMS using the Twitter SMS service.