To change which number your digital business card app is shared from using the Cardtapp native app (formerly Tapptracker), you will want to go to the Cardtapp app on your mobile phone.

Click on the hamburger navigation in the top right corner of the app

Click on "Preferences" from the Menu:

Click on "Mobile Marketing" tab in the menu

Change the "Send shares from" in the drop-down selector to "Cell Phone"