I'm drawing a blank here. Where can you change the setting on a phone for the number of rings before a call goes to voicemail? Is that in the phone itself, unity or call manager? Thanks.
If you are using Callmanager (later than CCM 4.1) you can change the number of rings before FNA to voicemail using this setting via ccmadmin.
(under>Device>Phone>Directory Number Configuration)
No Answer Ring Duration (seconds)
If you want to change this "system wide" let me know as this can be adjusted as well.
Hope this helps!