From Zenitel Wiki

Redboot is the bootloader used on the AMC-IP system. The bootloader does a bit of the same as the BIOS on PC. The system enters the bootloader at reset. If not interrupted, it continues with loading Linux. Once Linux starts, Redboot is not used any more. Redboot has functions for loading image-files to flash partitions. This is used if the Linux kernel or the complete root file system needs upgrades.

Redboot is a open source project :

Zenitel has extended Redboot with the "fis alpha" commands, which streamlines loading new kernel/rootfilesystem images. See AMC Flash Image update.