Appointments+ SMS Integration

I am using some code provided in the forum by a user and need to work out if something has changed in Appointments+ that would stop it from working.

The code that I tried as a mu-plugin is mentioned here - https://*****

The code was adapted from this response -

I just want to know if this refers to the right names of things in the plugin. I have installed the code as mu plugin and the plugin is running on the site, but the sms is not being sent.