I tried and failed to use the Blesta API - most likely due to my ineptitude.
Join is an SMS app, that lets you send text messages using a simple URL, the smart thing they have done is to provide an API builder, I login to my account, then from https://joinjoaomgcd.appspot.com (the management console), I input the destination phone # and text I want to send. Join outputs the syntactically correct url which I can use verbatim or edit as appropriate. Very helpful!
Join is described here: https://joaoapps.com/join-sms-from-anything-with-a-browser/
To test this, install the app on your phone and attach it to your Google account, then go to https://joinjoaomgcd.appspot.com/ and select the "Join API" button