As said, that may not be the hostname you need, or the only one you need. My server's hostname may be "bob" but for some reason the SIP registrar FQDN can be chris.example.com, i.e. its hostname part of the FQDN for SIP extensions, including Workplace, doesn't have to match the server's hostname (not sure why you would want it to differ but people do weird things).
The IP Office security guidelines doc has a list of which parts need to be in a certificate's SAN different clients. I found an error in that list so was waiting to talk to some folks tomorrow, but the implication is that both would be needed (the server host name as part of a FQDN for the server and the specific SIP registrar FQDN).
Stuck in a never ending cycle of file copying.