Open a known bootable Cisco ISO (even an older version like 10.5) and extract the isolinux.bin file from the Inject into New ISO: Open your non-bootable CUCM 12.5 ISO in the ISO editor. Set Boot File:

She downloads the full bootable ISO image ( Cisco_Unified_CM_12.5_SUx_bootable.iso ).

sudo dd if=UCSInstall_UCOS_12.5.sgn.iso of=/dev/sdX bs=4M status=progress