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Hissing on PSTN Calls

CM 3.10, 2621 as GW with HDV/MFT-T1. 16DS0 as E&Mwinkstart. When a call inbound or outbound via the gateway, a background hissing is heard. It is louder when no voice and quiets during voice. I have tried "no comfort" on the voice port and turning VAD off on dial-peers, but nothing seems to make the difference. Noise starts when call is connected to PSTN. Happens during ringback also.

Any hints appreciated.

Community Member

Re: Hissing on PSTN Calls

We had the same problem and the following fixed it: We added VoIP dial peers on each gateway that referenced our extensions, for example: 1.. and 21..

The dots should be there, as you do not want to enter a VoIP dial peers for every extension. Let me know if this helps out.

Community Member

Re: Hissing on PSTN Calls

Also make sure that "SilenceSuppressionSystemWide" and "SilenceSuppressionWithGateways" is set properly on all CMs. These are Service Params

Community Member

Re: Hissing on PSTN Calls

This type of sympthom indicates the problem is probably related with gain configured somewhere on speechpath. Check default values of gain/ attenuation configured on router.

Try to use output attenuation to decrease the 'speech power' in one of the directions.

First try some drastic adjust like '8 Db', then reach the perfect level, adjusting it softly.

Don't forget that speech power levels change can affect echo canceler machine.

The topology of the voice network should define were you must change gain/ attenuation config.

You can also try to contact PSTN to verify the voice switch configuration.

IOS command:

* "output attenuation <>" and "input gain <>"

Community Member

Re: Hissing on PSTN Calls

We had a similar problem using ISDN PRI's and the TELCO worked with the gain at their end.

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