You cannot download without an active service contract tied to a PID (Product ID) like MCS-7825, UCSC-C220, or a virtual CUCM entitlement.
Once downloaded, installing CUCM 12.5 is straightforward but requires planning. Cisco Cucm 12.5 Download
You will not see a download link unless your Cisco account is associated with a valid service contract for this specific product. You cannot download without an active service contract