What's the phone identifier (six character phone id)?

iPhoneModem registration is based on your iPhone's UUID*.
On the registration screen you'll find an identifier code defined by the last six characters of the UUID.

A separate registration is required for each phone running iPhoneModem. If you loose or replace your licensed phone, you can transfer the license up to three times.

* UUID stands for "Universally Unique IDentifier". Each iPhone has a different 40 alphanumeric UUID code.