A low hissing sound is heard when outbound calls are placed from a Foreign Exchange Office (FXO) port through the High-Density Analog voice or fax Network Module (NM-HDA). The hissing sounds as though the noise floor has been raised incorrectly. This symptom does not occur if the two-port voice Network Module is used instead of the NM-HDA.
Note: Slips can also cause echoes and hissing.
To resolve this problem, issue the nonon-linear voice port configuration command to disable non-linear processing on the echo canceler. Echo may still be observed after this workaround is implemented.
For a list of integrated releases, refer to Cisco bug ID CSCdw77268.