Along with an email address, the buyer usually has two numbers showing on the order details page. One associated with the delivery address and one associated with the account. 

They are not always the same, and sometimes there is only one number rather than two - but both an email address and at least one telephone number are always included. It is and remains a requirement for every eBay member to provide an up-to-date contact phone number. 

Saying that eBay doesn't require a phone number is simply not true.

I can't recall one single occasion where I've posted any item without a phone number on the label - letters included. I can, however, recall dozens of times when I've reported buyers for having false contact details.