CUCM 15 requires a minimum of ESXi 7.0 U3 or 8.0 U1 . Older versions like 6.5 and 6.7 are no longer supported.
To appreciate CUCM 15, one must first understand the constraints of its predecessors. CUCM on bare metal (versions 8.x through 12.x) was a study in static resource partitioning. A physical server dedicated to the Publisher node had CPU cores and RAM that sat idle 80% of the time, reserved only for burst call processing or database replication storms. Disaster recovery was a blunt instrument: a cold spare server that required identical hardware, identical firmware, and a manual, time-consuming rebuild of the OS from a Recovery ISO.
After completing the deployment, the team experienced several benefits:
This specialized edition of NFVIS is also supported from 15SU4 onwards, primarily for on-premises calling applications on select Cisco appliances. 2. Virtual Machine Resource Requirements