Looks like you clicked on the "I'm Mobile" link from Yahoo! Messenger.
What does "I'm mobile" mean?
When a friend's status says "I'm mobile," they are using a mobile version of Yahoo! Messenger and can send, receive, and reply to instant messages from their phone.
Get Messenger for mobile
Enter your mobile number.
Or type this URL into your phone's browser:
m.yahoo.com/messenger
Here's the easy way to get Yahoo! on your phone.
Want Yahoo! Messenger for your phone?
Stay connected with your IM friends from your phone—it's easy to send and receive Yahoo! Messenger instant messages when you're mobile. Learn more.
Yahoo! Messenger for BlackBerry®
Get Yahoo! Messenger designed just for your BlackBerry—it's one of the best mobile Messenger experiences available.
Get it here.
Yahoo! Messenger for iPhone™
There's a new, enhanced iPhone version of Yahoo! Messenger available now, at the App Store in iTunes®.
Sidekick users: you already have Messenger
An awesome Yahoo! Instant Messenger experience comes with your Sidekick phone, so pick it up and start chatting. On your phone's home screen, select the Instant Messaging category, choose Yahoo! Messenger, then enter your Yahoo! ID and password.
Get Yahoo! Messenger via SMS/text messaging on your phone
Any phone can send and receive instant messages through an existing text messaging plan. See how to get Yahoo! Messenger for SMS on your phone. Learn more.