If you have a valid subscription for licensing you should be able to download the bootable ISO file for a specific portal at Cisco. At the moment I can’t remember the name of it. Hopefully someone else can help you out with that if you don’t find it.
If you create a bootable ISO on your own you void all TAC support, so I wouldn’t do that.
Making the non-bootable version, if I recall correctly, works on 12.5 but not on versions 14 and above. This is not a recommended process, and no one can confirm if you will encounter issues or not—you need to try it yourself. If you do run into issues, it's uncertain whether Cisco TAC will support you.
You take ownership of any issues that occur using the bootable version you create from the non-bootable one.
CUCM 12.5 reaches EOS on August 31, 2025. If you have a valid contract for CUCM, reach out to TAC and request the files for installing the subscriber.
The MCE portal is where you can find the bootable files for installation, and @Jonathan Schulenberg has shared the link to the portal.
I'll try the option of installing the version for which I have the bootable file for 12.5 and then upgrade during the installation. Most of the time, this has encountered issues. This would be an option if you don't have TAC support an alternative will be making the bootable version you create from the non-bootable one.
If you don't have active support, I recommend obtaining it by moving to Flex. Upgrade to versions 14 or 15 and then add the second subscriber.
On your FTP or SFTP server put the upgrade file(s) in a directory that only contains these. Do not use your Download directory.
For the CUCM node you installed using Bootable_UCSInstall_UCOS_12.5.1.10000-22, to recognize the file UCSInstall_UCOS_12.5.1.18900-40.sha512.iso as a valid upgrade, you need to have the COP files or be on version 12.5.1.12013-1 or above.
Can you access the FTP or SFTP via a normal client with the same credentials and if so do you see the upgrade ISO in that directory? Second thing, do you happen to have any other files than CM upgrade ISO/COP files in the directory set as used by the FTP/SFTP server? If you do please move the file(s) into a directory that only contains the CM upgrade file(s) and update the configuration in your FTP/SFTP server software.
As @Maren Mahoney mentioned, you can install the node with Bootable_UCSInstall_UCOS_12.5.1.10000-2, then either upgrade to 12.5.1.12013-1 or install the COP file (if that works), and finally upgrade to 12.5.1.18900-40.
