I'm not sure about previous versions but in Magento 2.2Magento 2.2 you can go to
Store > Configuration > CUSTOMERS > Customer Configuration
Store > Configuration > CUSTOMERS > Customer Configuration
Then go to the NameName and AddressAddress Options tab on the right and look for Show TelephoneTelephone and select your option. (No) (Optional) (Required)(No) (Optional) (Required)
It's set to Required by default.