Not sure if Andrew is willing to part with his scripts but you could buy a working version (not sure if it unlocked).
We do this without plugins using a web based service from www.textmagic.com, although there are many similar services available. We buy SMS credits from them in bulk, usually 1000 at a time but they offer different discounts based on quantity.
In terms of integration, they (and quite a few SMS providers) have various options: -
HTTPS API - using a FM Web Viewer object we can send a query to their servers eg:
Obviously we use calculations to replace the message text and the telephone number and the page this generates in the web viewer can be queried for an acknowledgement. There are also additional functions you can query which return credit balance etc.
They also support email to sms (which we have found to be easier and faster). We use the 'Send Email' script step with an address like "email@example.com" subject="<password>" messagebody="<the message to send>". Using that method we can send the same message to 100's of recipients (we are a school emailing out to parents about trips and such) with a single script sending a single email with multiple recipients.
If you're in a position where you know the service provider for each number (such as for sending messages to employees on company-owned devices), you can use the provider-specific SMS gateways to send via email with the Send Mail script step without having to pay to use a more general gateway service.
I found a way to send record specific SMS's from FileMaker via an Applescript to Apple's Messages.app.
However, it only works if the receiver is known in Messages.app.
I still haven't found a way to send a SMS to "unknown" receivers.
Let me know if you're interested in what I found out so far (calculation fields, FMP script, Apple Script) for Apple users.
I originally was cheap and did text messaging via the various carriers email conversions. And that worked, but then I had a client with a need for a serious gateway and sms capabilities and the ability to send pictures and receive pictures. The picture thing is a big hurdle and basically we found out that doing so means you need one of the short numbers (e.g., 5 digit phone #) which all cost about $1000 a month. In my opinion FMSMS and one of the gateways is the way to go, but depending on what you need, things can get expensive with the gateway.