With the Automatic Post Expiration, I am able to trigger an email on expiration - client enters a time they expect to return from bushwalking, if they don't login to register they have returned, an email is sent to relatives etc.
My question: is it possible to also trigger an SMS text message?
No, it's not possible. I would suggest asking for this Feature here:
https://toolset.com/home/contact-us/suggest-a-new-feature-for-toolset/
Note that sending SMS with PHP (which is basically what WordPress is built on) should be easy when using some 3rd party service:
https://stackoverflow.com/questions/1939689/sending-sms-from-php
You could apply that in a Forms API hook such as cred_save_data, in there you simply would craft that request to the 3rd party and they send the SMS.
https://toolset.com/documentation/programmer-reference/cred-api/#cred_save_data
Or you can even use the email notifications, I assume, as explained in the StackOverflow linked above.
They suggest this service:
hidden link
It seems free, so maybe this is something we could document in Toolset, even, if you confirm this to be what you looked for and solves your goal?