Hi Ton
The phone number is not a required field in DNN, but for ecommerce we feel it's important to be able to contact the customer in case of payment problems or disputes.
If you don't want to require the phone number, you can try to edit the custom display template and remove the required validation and modify the JS to set the phone value to a dummy value such as "111-111-1111" before submitting. If you need help modifying the template, you may engage us to do the work for a small fee.
Thank you!