The fastest and cheapest way is to determine the email address for this person’s phone. Almost every cell phone has an email address to which you can send a message that will be displayed as SMS on the device. This will help you find the email address for each operator: http://www.makeuseof.com/tag/email-to-sms/
If you want to send an actual SMS instead of email, you can try using a service such as Twilio , which makes sending and receiving SMS from your applications easily through the REST API. [Full disclosure: I work at Twilio, but I have long been a satisfied customer before starting here.]
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