Learn how a flasher box works and what it does with the cell phone data. Familiarize yourself with the user interface of the software that comes with the flasher box. The “read” buttons are great, but there are also a lot of “wipe the data” buttons. There is only one way to learn all this and that is by using reference phones. Make a hex dump, change something, and dump the handset again. Look in the binary file for the differences and what they mean.
Bear in mind that no two flasher boxes are the same; that applies to the different handset brands and models as well. Even using the same brand and model, but with different firmware, the outcome can be different.
From: USB Port Monitoring and Flasher Boxes by Bram Mooij