The "official" way to get a bootable ISO is through specific fulfillment channels rather than the general support site:
Open the non-bootable ISO and locate the isolinux folder. Extract the isolinux.bin file. cucm 12.5 bootable iso download
** Cisco Unified Communications Manager (CUCM) 12.5 Bootable ISO: Full Report** The "official" way to get a bootable ISO
Previously known as the Product Upgrade Tool (PUT), this is the primary portal for customers with valid support contracts (SmartNet) to request physical media or bootable electronic downloads. cucm 12.5 bootable iso download
Use a tool like UltraISO to load the ISO and select "Generate Boot Info Table" under the bootable tab.