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ISR 4331 Voice Quality Issue - Hissing


We replaced a 2811 gateway with a new 4331 ISR router and voice quality is terrible.  Information  -


Code - isr4300-universalk9.03.14.00.S.155-1.S-std.SPA.bin

Voice Interfaces - Both T1 PRI and POTS line via FxO

CallManager Version 10.5, latest patch

Gateway Protocol - Issue occurs with both H323 and MGCP

Show Controller T1 shows no slips or errors for the PRI

Protocol - Both G.711 and G.729


The scenario is that voice quality on the IP Phone side of a call hears hissing and noise throughout the call but especially when the IP Phone side is speaking.  The hissing and noise can be heard as the call is being processed in the gateway.  Real weird.  We used a real blanket config (h323) and got these results.  Anyone else experiencing this?  Suggestions?





Greg Cording

Might be worth checking if VAD is in use during a call, VAD generates background noise (comfort-noise or hiss) to fill silent gaps during calls.



TCC_2 posted "How to confirm that VAD is enabled and how to disable it on Cisco IOS gateways" this some years ago..



To confirm whether  Voice Activity Detection (VAD) is enabled on a call-by-call basis, perform these steps:


  1. Make a call.
  2. Issue the show call active (brief) command.
  3. Check the values for the Voice over IP (VoIP) call leg.

To disable VAD, perform these steps:


  1. Confirm that the correct dial peer is being matched.
  2. Issue the no vad command under the VoIP dial peer.





Our resolution was updating the IOS on the gateway.  What version are you running right now?  I will get with our engineer to identify the exact version we applied.

The version we applied was - isr4300-universalk9.03.14.02.S.155-1.S2-std.SPA.bin

I wouldn't say our issue is so much a hissing, more like garbled voice when one party is talking.  Its incomprehensible what they are saying.  However it seems to match the other changes we made, going from 28xx router to 4331 with the new FXO card.

We are currently running:

boot system flash:isr4300-universalk9.03.13.05.S.154-3.S5-ext.SPA.bin

Cisco IOS XE Software, Version 03.13.04.S - Extended Support Release
Cisco IOS Software, ISR Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 15.4(3)S4, RELEASE SOFTWARE (fc3)

I'll try a new IOS and report back.  On a side note, what are all the additional files when I do 'show flash:'?  Are they necessary files for the newer XE OS?


I have the same problem, even using "no vad" command.

What I know is that some old Cisco phones like 7940, doesn't support G722 codec.

 If this codec is forced on your CCM, you have to apply on your router a profile to forcé other codecs supported by old phones:

voice class codec 100
 codec preference 1 g729r8
 codec preference 2 g711alaw
 codec preference 3 g711ulaw

dial-peer voice 1004 voip
 incoming called-number .
 voice-class codec 100 

dial-peer voice 1000 voip
 preference 1
 destination-pattern .T
 session target ipv4:XXX.XXX.XXX.XXX
 voice-class codec 100 
 voice-class h323 1
 dtmf-relay h245-alphanumeric

no vad

Lucas Phelps

Did you guys ever get this resolved? 

I replaced an old 2800 gateway ISR with a new ISR 4331 yesterday and now outbound voice calls to PSTN via FXO cards is terrible/garbled.   Inbound calls to gateway sound just fine.

Please help!  Thank you

Rob Huffman
Hall of Fame Community Legend Hall of Fame Community Legend
Hall of Fame Community Legend

Hi Lucas,

I'm sure "mzajeski" is most correct here (+5)

You may likely be hitting this bug;

Hissing noise can be heard prominently on .sout. The hissing noise is consistent while the called party is not yet answering then it comes in and out on the entire conversation. It can be heard most of the time when calling party is not talking. "No vad" and "no comfort noise" are configured on the voice port and VOIP dial-peer. This was experienced by the customer on his environment and was replicated at Cisco lab. There is a POTS line connected to NIM-4FXO on a 4331. DSP type, DM8147 and SP2700.

Platform -
cisco ISR4331/K9 (1RU) processor with 7831854K/6147K bytes of memory.
System image file is "bootflash:isr4300-universalk9.03.13.01.S.154-3.S1-ext.SPA.bin"

Voice Card -
"NIM-4FXO Voice Analog Module"

DSP Type-
DM8147 DSPWare 37.3.4

use hidden command"non-linear mute" under voice-port configuration.


HI cisco isr 4451 quality for voip, ivr and dtmf and digit collect is far behind cisco 3945 router also. Just with 10 calls the quality is so bad ,, we are looking to move back to cisco 3945

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