SMS or text messaging is a convenient way to send brief text message directly to a friend’s mobile phone. A text message can be up to 160 characters long or about 25 words. SMS is a convenient way to keep in touch with friends and family.
Yahoo! doesn’t charge you to send SMS, however, the person receiving your message will be charged standard messaging rates for each SMS received or replied to.
Yahoo! has partnerships with various wireless carriers to provide two-way messaging capabilities. For other carriers, you can send a message but the mobile user will not be able to reply.
Make sure the number you entered is a valid mobile number with a valid area code and phone number.