Are customers required to provide their phone number to place orders?

To help maximize conversion and prevent phone number entry from deterring customers, the Phone field is not a mandatory field. 

To encourage customers to enter their phone number, add text to the check-out process. Here are the possible methods:
 
Method 1:
Add a message to the Shopping Cart Message - Below the Shopping Cart message section informing the customer to provide phone number while checking out:

1.  Navigate to Content > Message Editor.
   
   
2. Add your message to the Shopping Cart Message - Below the Shopping Cart field and click Update.
   
   

Method 2:

Alternatively, explore the setting that enables or disables the phone number validation on the storefront:

1. Navigate to Settings > General Settings > Display Settings.
   
   
2. Select Yes in the Enable Phone Number Validation in Storefront drop down menu.
   
   
3. Click Update to save the changes.
   
   Note: This setting makes the phone number a mandatory field.