If you call another phone who's either re-routed or forwarded to voicemail, and attempt to leave a message, does the NuPoint "hear" the audio and save the message?
Are you absolutely positive that the phones are on the 192.168.10.0 network and not one of the other ones?
I've seen this issue before, but the phones were on a different network than the NuPoint, and that network wasn't included in the "local" network scope of the NuPoint's MSL. I believe the NuPoints subnet had been set to 24 but should have been 16 to allow for communications from all the different phone subnets. Because the call setup happens between the MCD and the NuPoint, it would setup correctly, but then when it comes time for audio, that flows directly from the phones to the NuPoint, and the NuPoint didn't recognize the phones subnet as authorized.