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New Member

Crackling in background on ip7940 phone

Symptoms>Pick phone up with a dial tone>crackling

Dials number >crackling

Ringing > crackling

Other ends picks phone up, normal no crackling and speech normal.?


CCM Version 4.2(3)sr3a

Phone 7940

1 line


Switch details>Cisco IOS Software, C3560 Software (C3560-IPSERVICES-M), Version 12.2(25)SEB4, RELEASE SOFTWARE (fc1)


This started when we put customer from traditional to ip system.

Any help appreciated and any questions required I will answer as best I can.


Re: Crackling in background on ip7940 phone

Crackling is sometimes caused by a defective power supply or some kind of strong electrical interference close to the phone. Try changing the power supply and moving the phone to a different location.

Recognizing and Categorizing Symptoms of Voice Quality Problems:

New Member

Re: Crackling in background on ip7940 phone

Thanks for that, am going to try power blocks and take POE out of the equation.



New Member

Re: Crackling in background on ip7940 phone

Is this happening to all the phones in the site or just 1 or a couple?

If it's the whole site, it may be the actual communication path between the phones and the call manager. I'd check the CCM switch-port config and double-check the VLAN (and Inter-VLAN routing if any) & the QoS.

I've said this because CCM actually interacts with the phone when the call is being setup (off-hook, ringtone, dialling and connecting with the other party), but once the call is connected, it's just an RTP stream between both phones.

If it's just one or two phones, then as dumb as this may sound, I think you should try replacing the phone itself.

I've had that issue in the last couple of weeks at a few sites.

What I found was that the port connecting the handset to the phone itself may be loose, or the port that connects the IP Phone to the switch; so a replacement phone did the job.

Hope you fin this helpful.



New Member

Re: Crackling in background on ip7940 phone

Any cordless phones from the Old PBX in the area, they will kill quality on IP Phones