TextBelt (http://textbelt.com) is a REST API that sends outgoing SMS. It is available as a public service or as an internally self-hosted server.
Textbelt uses carrier-specific gateways to deliver your text messages for free, and without ads. The service is fairly reliable and has sent over 1 million texts.
This appears to simply be taking advantage of many carriers’ existing email-to-text gateways. You can get this in HASS right now by using the SMTP notify platform and setting the recipient to your number @ your carrier’s gateway. Example for T-Mobile US: [email protected] . You can find your carrier’s gateway address in many places, one of them being TextBelt’s code.