If you cannot get access to the official ISO through a Cisco contract, consider these legitimate alternatives:
CUCM 12.5 has multiple COP file requirements. The base ISO (e.g., 12.5.1) will not work on UCS M5 blades with latest BIOS unless you also integrate a bootable COP file for drivers. Always read the "Readme" accompanying the ISO. cucm 12.5 bootable iso download
Path:
Products → Unified Communications → Call Control → Unified Communications Manager (CallManager) →
Unified Communications Manager (CUCM) Version 12.5