We run CUCM in a shared tenant environment. We have had various reports from clients complaining about static on the line. The problem seems to only be with AT&T cell phones. We are able to replicate the problem every 10 or so calls. The problem occurred through one carrier (we had a PRI hand-off with them), but their support was less than helpful. We have since brought in a new PRI and the same thing is occuring. We have tried terminating the PRI on different equipment (Cisco 3745 and Catalyst 6506 8-port voice/conf blade).
Is there anything from the Cisco IP phone to our carrier that could introduce static on the line? We are digital all the way through to the carriers switch: VoIP -> PRI to switch. I'm sure I know the answer to this, but a few clients requested validation. We have a TAC case open as well to confirm.
I also need to add that the static issue is one-way. We can hear it on the Cisco Phone, but the cell phone cannot hear any static. Also, we have silence supression disabled in CUCM and "mgcp vad" is NOT present on the gateway.