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Walkthrough Wednesdays

SCCP Terminated on CUCM to Asterisk via SIP Trunk - Garbled/buzzing noise

So I have a weird one. I'm just goofing around... I'm a route and switch guy, but have recently discovered the world of telecom and collab - WOW different world...

Anyway... I have

CISCO 8865 & 3 7920 SCCP wifi phones --> CUCM --> SIP Trunk --> Asterisk --> PSTN

Here's what's happening. Calls between SCCP and 8865 work fine, clarity is great, no problems.

Calls from 8865, jabber, between asterisk sip and cucm sip phones are all great, no issues.

Calls from SCCP phones to Asterisk or PSTN (via asterisk) give me a garbled/buzzing (almost "national emergency" TV warning) sound.

I tried turning on MTP on the trunk to asterisk... behavior changes in that now all calls between CUCM and Asterisk (or PSTN via asterisk) have the same garbled/buzzing sound. Turn it off and everything's good again.

I've searched and searched and can't find anyone else with the same problem. Maybe my google kung-fu is not so strong... but usually I can find the answer...

Anyone else ever have this problem (or similar)?  I'm at a total loss... I was wondering if it was maybe a codec issue... but then why would it work locally through CUCM? They must be negotiating the same codec...

Any help is appreciated!


Mohammed al Baqari
VIP Advisor


During the calls from sccp to pstn, can you check if cucm is already invoking an MTP using the command 'show sccp connection' on your mtp.

Also, during active call using mtp, can you issue the command 'show call active voi br' and 'show voip rtp con'

Thanks for getting back to me. I went through the sdl logs and verified that CUCM appears to be allocating an MTP channel when connecting to the asterisk pbx. unfortunately the show commands you suggest don't work on CUCM and asterisk doesn't seem to have an equivalent. However, that seems to conflict with the fact that the SDP exchanges are still connecting the end points directly via RTP.

I've been fiddling with a bunch of settings, but can't for the life of me get it working. I did find this link to a cisco article that gives an example of the exact noise I'm getting -

One thing that I found interesting was that the 7920 wireless IP phone is supposed to support g711a g711u and g729. But in the sdp exchanges, even when I try to force (via MTP and preferred codec on the trunk for example) 711A or 729, it only ever offers ulaw. never alaw or 729. I'm beginning to wonder if it isn't some weird codec issue...

Anyways, any further help anyone can provide would be great. I know I'm not providing a lot to go on...

Thought I'd reply with what ended up being the solution to the problem... although I'm not entirely sure why. I'd love it if someone were able to shed light on the why.

Also, if no one guessed, this is a LAB environment, not PROD.

Turns out that MTP doesn't like running in a KVM virtualization environment. Turned up a UCM instance in VMWare, did a backup/restore of my KVM config, and voila... no more buzzing/motor noise on the line.

I wonder if it has something to do with the resource reservations in VMWare, whereas I've not found a way to do the same (i.e. reserve 3.3Ghz CPU, etc...) in KVM. It's also different hardware, but I can't imagine that being an issue as they are comparable in terms of RAM speed and processor speed.

Any how, on the very improbably chance someone else attempts the same thing...

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