I had the exact same Problem.
You need an Android Emulator for your PC, since Whatsapp for desktop always needs to be paired with a phone, then install Whatsapp Business. The Business Version lets you sign up with a landline number. Youll geht a call with the code read to you and youre done.
If you already have a number and are planing to keep it, you can just use that and read the code from your phone.
If you decide to use a desktop app or the web version instead, you will have to open Whatsapp on your phone once every two weeks or it will unpair.
Opera 9.0 released June 20th 2006