Intermittent Dead Air on Cisco Phone

Hi All,
I have users reporting that midway through a call that they suddenly get dead air. Placing the call on hold, and retrieving the call re-establishes voice in both directions.

Model: Cisco 7970
Firmware: SCCP70.9-1-1SR1S

This phone is an Agent taking calls from UCCE. The issue only happens a few times a day, and has happened on different phones at different times.

Wondering if this could be a bug perhaps?

There is no routing issues or firewalls in play here.

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Maybe some spanning tree recalculation?
mihailpetreskiAuthor Commented:
I'll try and get some more information today.

I'm planning to leave wireshark running on a users machine, this user often talks over 15minutes so hopefully i catch the dead air issue again.

I'll post back soon with my findings.
Some news about this issue? did you solve you problem or have further information to provide?
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Hi mihailpetreski,

I did encounter this behaviour previously - the cause ended up being a network issue - packet loss was occurring.
When I encountered the issue, it wasn't just the voice stream that was interrupted, in some cases the control information was lost also - e.g. a shared DN would be delayed ringing on some phones etc.
If you manage to capture packets at the time of the issue, you may see TCP retransmits if control information is lost.
For us, there was a spike in CPU utilisation on the switches at the same time, which was mainly due to a bug in the IOS. Upgrading to the latest available version did help matters significantly.

To check CPU utilisation on a Cisco switch, you can run this command from Privileged EXEC mode
show processes cpu history

Hope this helps. Would be interested to hear the outcome!
mihailpetreskiAuthor Commented:
I did check CPU a few days ago, the last 72hours doesnt have an indication that it went over 20 -30 percent. I'll verify again when i am informed of the issue.

Also managed to capture some wireshark logs from the time of an error, will need to get it off the user today and have a read through.

Packet loss seems likely, will have a dig and let you guys know when i find something. Or ask a question if I need anything :)
mihailpetreskiAuthor Commented:
I finally corrected this issue a while back but forgot to reply to you all.

I extended the SIP Session Exipires Timer in Service Parameter Configuration. The default was 1800 and I currently have it at 2500.

I found that it was getting dead air at about the 15min mark.
1800 / 60 = 30 min, however this is treated as 2 to 1 so it's actually 15mins (this was just trial and error to work that part out).
As soon as i extended it beyond 1800 I never saw the issue again. I put it back to 1800 and sure enough i got the issue again.

I opened a Cisco TAC case to find out if this was a bug but Cisco was just as surprised as I was that this seemed to fix the problem.

Thanks for your assistance.

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mihailpetreskiAuthor Commented:
The issue ended up being a SIP session expires timer.
Cisco TAC is unaware of why this would be causing dead air at 15minutes - it is currently with the Cisco developers.

I found this issue by trial and error in my lab enviornment.
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