If you’re seeing the Company Code request screen when setting up the App add-on, that means you’ve made a typo in your mobile number either in the Control Panel or in the App.
Please double check the mobile number you’ve invited to install the App on to ensure it is correct and there are no typos in the mobile number you’ve entered in the App page of the Control Panel: https://cp.airlandline.co.uk/account/voip/apps/
Also when you’re setting up the App, you need to enter the same mobile number during setup (not the landline number).