The latest hurdle I’ve seen involves moving existing clusters to FQDN instead of IP. This usually involves making sure all endpoints registering to CUCM have DNS servers and a search suffix provided by DHCP.
Every installation I’ve done has been based on server name and DNS. One client had to change an installation back to IP in the server field because of their DNS domain hierarchy. Heading into DNS usually involves triple checking the DNS/DHCP side of the world before changing a cluster. For a long time Cisco TAC even advocated that cluster server names are set to the IP address. I’ve always had interesting conversations when I would open a TAC case on of my installations.
With the introduction of Jabber, certificates (and multi-SAN in 10.5), and IPv6 we finally have come full circle to needing the server set to FQDN. Since you cannot apply an invalid TLD or IP address into the subject or SAN field names you’ll need to get certificates based on an actual public FQDN. If you want full BYOD interoperability the certificates you get should come from a 3rd party valid certificate authority. (I like Digicert)
Long story short – your “CUCM > System > Server” should be set to hostname.domain.com