Your customers can tap the button and their phone will offer to dial your number.
Click on the Widgets tab (1) and you should be able to see the Call me widget on the right (2).
Select the Call me widget and drag it to your page (1)
Click on the widget so you can see the settings tab on the left.
1. Enter the phone number
2. Add the text you want to appear in the button and change the style of the widget
3. Align the widget
4. Don't foget to save your changes