The SMS feature lets you send text messages to your friends' mobile phones right from Yahoo! Messenger. Just type in a friend's mobile phone number, compose your message like you would a regular instant message, then hit Send. Your friend will receive your text message on their phone, and their replies come back to you in Yahoo! Messenger.
It's free to send SMS messages from Yahoo! Messenger to your friends (and to receive their replies). However, your friends may incur data charges from their mobile carrier for receiving and replying to your messages.
Yahoo! Messenger supports SMS messaging for the following countries and carriers:
United States: AT&T, Verizon Wireless, T-Mobile
Canada: Rogers, Fido
India: Vodaphone Essar, Airtel, Spice, Escotel, Reliance
Indonesia: XL, Telkomsel, Hutch 3
Malaysia: Celcom, Digi, Maxis
Philippines: Globe, Smart
Thailand: AIS, DTAC
Vietnam: Mobifone, Viettel, Vinaphone