The most likely cause of the hissing during gaps in speech is comfort noise. Test by disabling comfort noise and Voice Activity Detection (VAD). For more information, listen to the Hissing Periods sound recording.
To resolve the issue, perform these steps:
Test whether comfort noise is causing the problem:
Select an appropriate VoIP dial-peer to test. Issue the no VAD configuration command under the dial peer.
An alternative to entirely disabling VAD is to disable comfort noise on the appropriate voice port. This is done by issuing the no comfort-noise command. Without comfort noise, VAD can cause absolute silence between speech. This can be equally annoying due to the loud hissing. In most cases, it is preferrable to turn VAD off.
Set the SilenceSupressionSystemWide and SilenceSupressionWithGateways parameters in all your Cisco CallManagers to false. Select Service > Service Parameters > Select Server > Cisco CallManager.
Next, attempt to tune comfort noise so that the transition from real background noise to comfort noise is masked as much as possible. Issue the voice vad-time global command to control the rate of this transition from rapid to gradual.
3. When satisfied that comfort noise is causing the problem, and if tuning the voice vad-time command does not decrease the impact of the problem, disable VAD on all appropriate dial peers.