"Verify Contact Information" in Apple Pay

I am trying to use ApplePay for the first time. I have a link set up with my debit card, and verified that with the bank. Now in the web page of the vendor from whom I wish to make a purchase, I clicked the Apple Pay option to display a Apple Pay dialog. That dialog shows my credit card info. But it has a red error message at bottom saying "Verify Contact Information". The "Contact" field has red letters "PHONE NUMBER REQUIRED". I have no idea whose phone number it wants, nor what I should do next.