To sign in to Yahoo! Messenger SMS, type "in [Yahoo! ID] [password]" on your mobile phone and send it to 1411. Your status message will appear to users on the PC version as "I'm on SMS".
Once you have successfully signed in, you will receive a list of your online friends.
If your friend list exceeds 160 characters, hit "Reply" on your mobile phone with the command "more" to receive the subsequent list of friends. Continue to reply to these messages with the command "more" to receive your entire friend list.
To sign out, type "out" and send.
You will receive a confirmation message acknowledging your sign out request.