If you navigate to the Call tab on the record of a contact who does not have a phone number, you will see No phone number next to the contact's avatar and the Call from... button will be grayed out and display Number invalid.
You can quickly add a phone number for the contact. To do so, click the No phone number dropdown and select Add phone number for their phone number of mobile phone number property.
Input a phone number and click Save. This will update the Phone number property for this contact. This dropdown also provides the option to use the phone number for the contact's associated company, if applicable.