I have noted this problem that when a subscriber has been left a message, if there is a pause for more than a 1/2 second, the voicemail will cut off. Is there any way to extend this time frame for no noise on a message or is this a CallManager issue?
Half a second pause out? Really? That's not right if that's even close to accurate. A half a second is amazingly fast for cut off time.
by default Unity will wait for 2 seconds of silence on a short message (less than 10 seconds) and 3 seconds of silence on a long message. These are both adjustable on the "recordings" page in the Configuration section of the SA assuming you're at some reasonably modern version of Unity (say 3.1 or later).
Be careful adjusting them up too high. But if you really are getting half second cut off times then something else is afoot since no timeouts in Unity are even close to that.