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Static issues

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.


Re: Static issues

Hi Jordan,

From a troubleshooting perspective - if you've recreated the problem on a 3745 as well as a 6608 blade, there's almost no chance the Cisco gear is causing the problem. Talk about apples and oranges.

Static is an analog/DSP caused problem. You should not be creating any on a PRI, and this doesn't sound like it's an issue on the DSPs since you've tested entirely different versions/hardware.

What is very possible is a certain channel on your PRI. Sometimes it's helpful to place a call, test the line, put it on hold, and try another. This fills up the PRI and allows you to test some of the channels that are not used until the PRI is filled up. You can also busy out channels or work with the provider.

Command for a non-mgcp controlled PRI:

interface serial 0/0:23

isdn busy b_channel

'show isdn service' tell you which ones you have busied out.

Cases like this can drag on, because we're fairly confident that problem is on the provider's side, but the provider will send a guy out, he'll make a couple test calls that sound good, and say it's a problem with your equipment.

The only time you should get static is when the RTP packet is created or modified, and generally that's only going to happen on your gateway and IP phones.

If you're working with TAC, you can get a PCM capture during the call to prove it's the provider.