Phone/call button

Hello, I’m using an Airtable formula field to create a call and mail column:

In Softr Studio I’m using a button with action open external URL mapped to these columns. The email and Google Map button works, the phone one ends up URL/undefined.

Am I doing something wrong or is there another approach to a call button?

Is the idea to have the call button launch the dialer on a phone? What would happen if the user were using a computer?