SMS are not sent using workflow

Hi

I have configured workflows to send SMS when creating an event and as an reminder.

When a client is booking a meeting, then this works fine.

But when I‘m selecting a client directly in my calendly and adding an event for him, then only the standard-email notification is sent, no SMS. 

What can I do to ensure that workflows are reliably triggered and SMS messages are sent?

Thanks for any ideas

werner

PS: I‘m using the standard plan