Checkout on woocommerce cannot be completed

Hello,
I have set the payment method of woocommerce only by PayPal.
But at the checkout, the PayPal does not accept without having filled a phone number.
But it is not possible to add a phone number afterwards, at the checkout page.
The only solution is to back to the form and fill out a phone number.
As it is optional, phone number to fill in the Checkout form of the template, many people do not give their phone number, and they get stuck at the checkout page.
It is better if it is written with Asterisk that it is required to fill the phone number, but I don’t know how to configure.
Checkout form and Woocommerce were not changed from the default setting of Woodmart.
Am I the only one who has such a issue?
I appreciate for any help.
Thank you and
Best regards,
Ichiro Nagata