App bug: App says scanned Serial number is invalid (with fix)
Just in case anyone else has this problem.
I Installed the Cell Booster app on Android. When I went to setup an AT&T Call Booster, I scanned the blue QR code on the booster. It entered the Serial number, but them told me it was invalid.
Quit and retried several times.
A close check revealed that scanning the QR code left off the last digit. After I added the last serial number digit manually, it let me go to the next step.