Step 1: Click on Checkout, then click on Behaviour and Edit Behaviour.
Step 2: Click on Behavior tab. Enable the Require Phone Number button, if you would like the phone number field to be mandatory at your checkout.
Step 3: Once you are done, don’t forget to click on Save changes
You can set up the phone number field to be mandatory at your checkout by clicking on the Checkout tab in the menu of your Admin panel of the selected store.