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.
Your carrier's standard message and data rates may apply. This number will only be used to send you an SMS text message explaining how to access Yahoo! on your mobile device.
Text HELP to 92500 for help.
Text STOP to 92500 to cancel.
Or type this URL into your phone's browser:
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.