Sorry, I don't have a controller in front of me.
I think it's in the controller System Settings (not CoS) and it's called something like "Multilingual Display".
Obviously "Multilingual Display" doesn't mean "Display the full first name instead of just the initial", but ticking this is in fact the solution for displaying full name for some reason.
Got me wondering so I looked it up...
System Options - Multilingual Name Display
Description:
Select "Yes" to enable the Multilingual Name Display feature, allowing User and Services Configuration (USC) Names, which include UTF-8 encoded, to be displayed on various endpoints within the cluster, instead of the Telephone Directory Names, which support ASCII characters only.
Notes:
User and Services Configuration (USC) Names will be displayed in full where space is available.
For SIP calls, you must also enable this option in the SIP Peer Profile and/or SIP Device Capabilities form.
Default value:
No