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intermittent call quality issue

gavin han


we have one site that's having intermittent call quality issues. it has 4 T1s bundled out of which 1 T1 has been taking errors. we did some test and didnt find call quality issue with T1 was taking errors. Please advise what the problem could be or any debug commands that could be helpful.


7 Replies 7

Christopher Graham
Cisco Employee
Cisco Employee

We need a bit more information as to your call flow.  You say that you have 4 T1's.  Are these PRI circuits?  To determine where we need to look, please provide the following:

- What is the actual symptom?  Static? Choppiness? Robotic? Echo?

- Call flow (eg: pstn--pri--gateway--h323--cucm--ip phone)

- Who hears the poor audio quality? PSTN or IP phone side or both?

- Is there a WAN connection between the gateway and the IP phone or CUCM? If so, check your

- Is there an MTP being invoked in the call flow?

If the issue is heard on the IP phone side, check the RTP stats on the phone during the issue (hit the ! or ? twice on the phone).  Are you taking any Rcv Lost packets?

If the issue is heard on the pstn side, 'show call active voice br' will show whether there are any lost/early/late packets.


4 T1s are data T1s that terminates to remote center where CUCM resides. these phones are analog phones.

analog phone<->pbx<->voiceGateway<--4 T1s data ckts-->CUCM<-->IP phone

actual symptom is static.

people on analog phone hears poor quality sometimes.

yes, there is a WAN connections as above.

No, I don't think if MTP is being invoked.

so, when I call them from my IP phone. i hear their voice pretty good but they hear static sometimes (intermittant).

pls advise

Hi Gavin,

Is it possible to collect PCM capture on the voice gateway ?

This will be used to isolate if the static noise is coming due to WAN or is it because of the PBX.

The steps are as follows:

  1. Enter configure terminal mode on the GW.
  2. Enter "voice hpi capture buffer 3000000" to configure the capture buffer.
  3. Enter "voice hpi capture destination flash:pcm.dat" to specify the destination file.


: If you will be making a large PCM capture or will leave it running for a long period  of time it would be best to direct to a TFTP server. To do so, use this command:

"voice hpi capture destination tftp://a.b.c.d/pcm.dat"

Where a.b.c.d is the IP address of the TFTP server.

Once you have the above configured you are ready to capture the PCM stream from the router.  To do so follow these steps:

  1. Start a test call and leave it connected.
  2. Enter "show call active voice brief" and determine what voice port the test call is using.  "show voice call sum" will also show what ports are in use.
  3. Enter "test voice port 1/0/4 pcm-dump caplog 7" where 1/0/4 is the number of the voice port that has the test call connected.


: If you are capturing a call on a PRI you will need to specify the voice-port number and the b-channel the call is on.  For example:"test voice port 0/0/0:23.14 pcm-dump caplog 7"

Where the voice-port is 0/0/0:23 and the b-channel, as shown in "

show call active voice brief", is 14.

Note: some versions of IOS require you to enter a duration command after "caplog 7". "duration 255" is sufficient.

When you are done capturing, do the following:

  1. test voice port 1/0/4 pcm-dump disable
  2. no voice hpi capture destination flash:pcm.dat
  3. no voice hpi capture buffer 3000000

Hi, Thanks for your reply. I've that captured file. but how to view it? opened it up with notepad and wordpad but it's shows unknown characters/lines in the file. created .txt capture file as well but got the same result.

thanks in advance

Hi Gavin,

Please attach the pcm file with this post and I will analyse it for you.


PS: Rate if useful

Hi, a quick question: is it safe to share it here? =(

im just worried because it's about our security.

The file only contains the audio.

You can again try and make a test call and make sure you capture that call.

Then this will be a just a normal call conversation. I don't think this will be any issue to your security.

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