ESXi customized OEM (Cisco in this case) versions are a point in time release.
Over time as new VMware bugs are identified and patches released, the customized images may be at a patch level that had an identified issue, and the customized image is no longer available with the expectation that customers will use newer patched levels.
ESXi 6.0.0 build -3873146 doesn't match anything on the VMware build list but looks like it is just above ESXi 6.0 Express Patch 6
I would take a look at https://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/uc_system/virtualization/virtualization-cisco-unified-communications-manager.html which lists the various UC applications, hardware, and ESXi versions, and go with a more recent version of ESXi that your UC application is supported on.
For instance, version 12.5 of CUCM, lists compatibility with ESXi 6.5 and ESXi 6.7.
I would probably go with the latest version of a Cisco Customized image unless the CUCM/call manager team lists some known compatibility issue with it.
Let's say you see that UC chart shows you can go with 6.5 or 6.7, and you decide to go with 6.5. You would go to https://my.vmware.com/web/vmware/details?downloadGroup=OEM-ESXI65U3-CISCO&productId=614 and download.
Once installed, you also should check the UCS matrix at https://ucshcltool.cloudapps.cisco.com/public/#, and pick the correct standalone server (i.e. C220m4/m5), the version of ESXi (i.e. 6.5U3), and confirm what drivers should be installed.
The image will have relatively recent drivers, but again, those are just a point in time image, and the HCL is updated as bugs or driver improvements are released. That HCL will also reference a Firmware level on the server, and the Firmware on these servers is updated via an ISO/bootable update image called a HUU. If you have a C220M5 based server, then you would go to https://software.cisco.com/download/home/286318809/type/283850974/release/4.1(1f)?i=!pp
Also, you can adjust the Version displayed on console with Esxi shell command: esxcli system welcomemsg set -m="Cisco business Edition 6.5.3 export Unrestricted Edition"
Kirk...
