Most phone companies provide a way to translate e-mail into SMS messages. Essentially, they set up special e-mail addresses; if you send e-mail to one of these addresses, the company sends an equivalent SMS message to the desired phone number. You can find lists of these gateway address by using you favorite search engine (e.g. Google, Bing ) and searching for "email to SMS gateways"
To send SMS messages to a particular user instead of normal e-mail, specify the appropriate e-mail address in the user's contact record. Since you can only specify one e-mail address per person, you can send either SMS or e-mail, but not both.