Daniel,
The productized SMS server application is designed to only connect to an SMS Gateway via SMPP. As far as I know we don't yet offer an SMS product to connect to vendors that only provide a REST API.
With that said, if you have the development resources to do so, you can write your own connector to those 3rd party SMS providers using Digital Messaging Server (DMS). DMS offers a Java-based plug-in interface that allows you to develop a custom media channel driver (https://docs.genesys.com/Documentation/ES/latest/DMS/CustomMediaChannelDriver). By developing a custom media channel driver, you can connect DMS to almost any messaging provider that has offers an API. I posted an article on the Genesys DevFoundry with an example that uses DMS to connect to Twilio SMS:
https://docs.genesys.com/Documentation/ES/latest/DMS/CustomMediaChannelDriver. The sample isn't production quality, but it would give you a good starting point to develop your own channel driver to whatever SMS vendor you choose.
By the way, did you know that Genesys now provides our own SMS aggregation gateway service? If not you should ask your Genesys account rep about it.
------------------------------
Jim Crespino
Genesys - Director, Developer Enablement
https://developer.genesys.com------------------------------