Whatsapp not only test the mobile IMEI numbers but also test the valid or wrong numbers.
suppose if you have entered a invalid/wrong number.
To test this,you need to have a mobile with SIM inserted and whatsapp installed and with minimum balance.
Then the procedure will be as follows:
1. Launch the application.
2. Enter any number other than your mobile number and tap on “OK” button.
3. Verification process continues and after 10 minutes of total time.
4. Your mobile gets “connection failed,reverify” popup.
This will happen because whatsapp will work for one number and one device.
If any one try to register with a invalid/wrong number,a highly secured procedure is implemented.
Understanding WhatsApp 