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IP Phones crackling on outbound PSTN calls



I have approximately 16 remote sites, each with their own local PRI (some with multiple PRIs) - we are just starting to receive complaints of crackling on the IP phones when initiating outbound calls to the PSTN. Internal calls are fine as well as PSTN calls into IP phones. This is happening at multiple locations therefore I beleieve we have a CUCM issue somewhere and not relatedto gateways/PRIs/slips as they all look clean. I can duplicate the issue on my IP Phone as well - when dialing there is no crackling but as soon as the call begins to connect and ring the crackling appears and is appearing more frequently on more phones at more locations.

we are running CUCM 8.6(2a) and the issues with quality have appeared and are occuring more frequently since we upgraded to 8.6(2a) aprrox 3 weeks ago (bug issue maybe??). Our phones (mainly 791x and 794x versions) are running sccp 9-2-1s. Is anybody running this version of CUCM and/or phone load and if so have you experienced similar issue with call quality? Our UC environment has been rock solid for years until this issue appeared

any ideas would be appreciated


7 Replies 7

Gregory Garrian

Is the crackling happening during the duration of the call?  You say you can hear it while it's connecting and ringing but you didn't specify if it's for the whole duration.

I ask because CUCM sets up the call but once it's connected the RTP stream (audio) goes from endpoint to endpoint.  In this case it would be local phones going out their local PRI.

Yes,it is the duration of the call - the callers on the remote side cannot hear the static/crackling, just our employees, if that helps. i plan to reboot the cluster (3 CUCM) this evening anyway but do not anticipate that resolving it

Thanks for the response

What were you running prior to 8.6(2a)?

Did that upgrade involve new firmware to the phones?

We were running CUCM 8.0(2c)SU1 prior and were issue free. I am curious to know if it could be the new firmware - we definitely had a different version on the older CUCM, i beleive it was SCCP 8-3-2. can i back down a version or two on the phone firmware?

It could be the firmware but since it's only affecting outbound calls I think it's unlikely (but not impossible).

I'd like to get some info on the gateways.  What protocol is CUCM using to communicate to the gateways?  h323, MGCP, SIP?

Are MTPs required on your gateway/trunk configurations?

If so, do you have MTPs configured on each gateway or are you just using CUCM?  If they're on the gateways are they registering properly?

If your gateways are set to require an MTP and the only MTP configured is the CUCM server then the RTP stream would flow through CUCM and that would be the common demoninator.  But, if that were the case it should afftect all calls, not just outbound.

As an FYI on the issue - on Thursday evening I wound up rolling the phones back to sccp 9-0-3 from 9-2-1s, rebooted the cluster to have the phones downgrade their firmware and have not heard a complaint since.

In addition to what protocol and firmware version is being used, it would be interesting to see what codec is being used.   I was using 9-2-1S and was having trouble with internal calls using G.722.   Basically as soon as a call was connected with that codec being used both parties received a busy signal.

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