Hissing at the start of calls


With our VoIP trunk any outgoing calls there is a 5 second noise then once it starts ringing it goes away.

I’m trying to figure out what it is. My provider said this “empty noise generated on the GS phones while it’s waiting a connection to finalize”. I’m not sure how to delay this as we don’t have issues with other phones. Seems to be only with the grandstream phones.

Any ideas?


As you mentioned phones…you failed to mention which models and firmware.

To be certain of the issue, you should do a network capture at the UCM while making a call that exhibits the issue and post the results. Please mask some of the public IP info, but not all as we need enough of the IP to know if public or private when look at the pcap file.

While I could be wrong on this, CN (comfort noise) is generated when the phone detects silence. In some phones it is tied to the Voice Activity Detection Circuit (VAD). When the VAD does not see activity it then engages the CN. There may be some settings within the phone that you can adjust to see if any change. This function causes RTP packets to be generated so that the other side will know that the connection is alive and won’t end the call prematurely thinking that the loss of packets is a sign of a dropped connection. It is my opinion that you would not hear any noise being generated by the phone as the packets are being sent out. So, there is either CN being generated by the provider, or sidetone or some other issue at hand. It may also be nothing more than the provider’s connection being setup which may involve other carriers beyond them and as they may have early media enabled, you are hearing whatever it is on the other side of your provider and there may not be anything you can do.

However. you mentioned other phones not having the issue,. what make and model are these?


It’s any phone we put on the UCM.

The current set up is 6208 current firmware with 2170 phones with current firmware.
Does the same on a 6202 with current firmware and 2135/2140 phones with current firmware.

First thing I did was update it to the current firmware to see if that would get rid of the sound.

It’s only on outgoing, and only the caller that hears the sound. As soon as it starts to ring, it goes away and you won’t hear it ever while on a call. Only when making outbound calls.


I bet you can hear the “noise” when you set up calls with G729.


I’d start super simple

If you can call extension to extension without the noise then it is likely SIP trunk or internet related.
If you call extension to extension and you DO have the noise, then it’s local to either the phones or the network.