Hello.
I have the exact same problem.
In the API documentation, there is an entry called SMS, where you can PUT a request to https://<ise-server>:9060/ers/config/guestuser/sms/{id}, but I'm not able to make it work.
When I put the user id into the {id}, I only get the message "Not found"
But even if the user was found, it means that we have to make 3 entries (steps) to create a user and send SMS:
1. Create user
2. Find user ID by submitting a GetbyID request
3. Send SMS using the found user id
Isn't there a way to send SMS automatically after the user is created?
In the self service portal there is, so why not here?