WhatsApp actually has two ways of API. The one in this example and a shorter one which is
One of them accepts leading 00 and the other one doesn’t. I’m not sure which is which but both of them requires country code with the number.
I check the length of the phone number entered if less than 12 (in my case) then I remind the user to enter both country code and phone number.
With business account you can actually listen to messages and respond to them. The business WhatsApp allows sending marketing messages to group of users, too.
I’m not sure if I answered your question or not but was just spreading out what I know