It is much easier to keep track of your MagicJack units if you know how to find their serial numbers.
The numbers have this format: A921050400442A-2304
1. Under the Menu button in the dialer, "greyed out" near the bottom of the menu.
2. Windows, Device Manager, Disk drives, Properties, Details, Device Instance Path, Value, Scroll right to see the code starting with A92...