The signal is being boosted at the voice gateway or, for Cisco Unity versions without Automatic Gain Control (AGC), the Cisco Unity-CM TAPI service provider (TSP) wave gain playback and record settings are possibly set too low.
To resolve this issue, perform these steps:
If only outside callers are affected, check the voice gateway gain and attenuation levels. Refer to the the appropriate documentation for your configuration in Cisco Gateways for instructions on how to adjust attenuation and gain. For Cisco IOS Software, use a positive dB value on the input gain command applied to the appropriate voice port to boost the signal if necessary.