We are looking to monitor the amount of voice connections to our voice gateways and tried to use this item but it returns 0 everytime. The device is a Cisco 2951.
Using the wrong vendor cert
In this case the backing vendor cert for the "Voice DS0 metric family" was "Cisco Voice DS0 Channel" The vendor cert priority list needed to be updated to use "Cisco CPM DS0" which contains cpmActiveDS0s - 188.8.131.52.184.108.40.206.220.127.116.11. After making this change you should be able to successfully report on "Connections"
Vendor Cert Priority List Documentation: