Hi Rob,
The above task is not complex, but I am not sure if the end-user license will have permission for doing this or not.
If your navision version is prior to NAV2009, then you need to make change in form no. 5050 (Contact Card or any other on which you want to change, for NAV2009 make the changes in Pages also), press Ctrl+F2,
check if the system allows you or gives any error?
If it allows, then copy the mobile no. control and label from 2nd tab and paste on first tab.