The restaurant doesn't need Whatsapp!!!!!!!!!!!!!!!!!!!!
You can call landline from Whatsapp. My friend called me on Whatsapp to my landline in Miami. She was in New Zealand.
Another called from Germany.
Restaurant doesn NOT need Whatsapp.
Do you know what VoiP is?

For calling GSM numbers and landlines, you need internet connection and mobile dialer on the originating side of the call.