I have a link with the API click to chat from whatsapp. The button with the link have the URL https://api.whatsapp.com/send?phone=5527998497670&text=Hello.
It works on mobile devices and desktops that have the application installed. For browser's users, the API open the Web WhatsApp but it doesnt open the contact linked in the URL.
I've tried to use //wa.me/phone_number too.
EDIT: I saw it in others posts that is a problema with whatsapp web server.