GSM7228PS and Cisco phones

We are trying to add Netgear GSM7228PS switches to our existing Cisco based network. Having trouble with Cisco 7900 series phones losing registration with the call manager server. We are uplinking the switch to a Cisco trunk port using PVID 999 and voice vlan 100 and data vlan 1. I have configured the Voice Vlan feature in the GSM7228PS. Here is the code I used

vlan database
vlan 100,999
vlan name 100 "Voice"
vlan name 999 "Trunk"
exit

voice vlan
class-map match-all ClassVoiceVlan ipv4
match vlan 100
exit
policy-map PolicyVoiceVlan in
class ClassVoiceVlan
assign-queue 3
exit
exit
interface 1/0/1
voice vlan 100
service-policy in PolicyVoiceVlan
description 'End_User'
vlan participation include 100,999
vlan tagging 100

Trunk ports are coded as follows

interface 1/0/23
description 'Cisco Vlan Trunk'
vlan pvid 999
vlan participation include 100,999
vlan tagging 1,100
exit

Can't figure out why phones register and then drop off. Any ideas?