Courtesy of Tim Dietric...
An SMS gateway address is simply an email address that maps to a mobile phone number. Nearly all carriers in the United States provide these addresses. (You might run into problems if you try to send a message to mobile users outside of the United States, however.)
For example, suppose that you want to send a text message to 804-555-5555, and you know that the carrier for that mobile phone is AT&T. The SMS gateway address would be: email@example.com Sending an email to that address will result in the message arriving on the user's mobile phone as a text message. I've put together a pretty thorough list of SMS Gateways here: http://timdietrich.me/fmauthenticator/sms-gateways.php
The problem is not in the sending of txt from FileMaker it is in the service provider for the gateway to the phone system. In the UK we have found them very unfriendly.
360 Works has Email 2. I have never used it but it enables sms messages from FileMaker Rich Carlton's a solution in a video that shows you how to build one natively in Fielmaker hereFileMaker Videos | Sending SMS Messages from FileMaker Pro no Plugins - YouTubehttps://youtu.be/raORZku7hb0
If you are intending to send messages to a select few people such as yourself, you could figure out your/their service provider, and send messages for free.
If you intend to send txt messages to many people / contacts / customers etc., you would need to know their service provider as outlined by Tim Dietric. Which is where you need either a plugin, as DonCollier mentioned - there are a few out there - or use a provider such as twilio.com. They provide a web API for sending sms messages and are pretty easy to implement . The plug-ins cost money up front, the web APIs charge a per-message fee, usually under $0.01