If the problem affects only messages from outside callers, the issue may be as simple as signal gain at the voice gateway. If all calls are affected, then for Cisco Unity versions without Automatic Gain Control (AGC), the Cisco Unity-CM Telephony Application Programming Interface (TAPI) Service Provider (TSP) wave gain playback and recording settings may be set too low.
To resolve this issue, perform these steps:
If only outside callers are affected, check the voice gateway gain and attenuation levels. For instructions on how to adjust attenuation and gain, refer to the the appropriate documentation for your configuration at Cisco Gateways.
For Cisco IOS Software, use a negative decibel (dB) value when issuing the input gain command that is applied to the appropriate voice port to boost the signal if necessary.