The signal strength is low on egress from a Cisco IOS voice gateway or the gateway is artificially lowering it.
The signal strength can be raised at the gateway by configuration commands.
To resolve this issue, perform these steps:
voice-port 0/0 input gain 1 output attenuation 2
input gain 1
output attenuation 2
The shut and no shut commands are required to accept the voice port changes.
For more information, refer to the Configuring Voice Ports section of Cisco IOS Voice, Video, and Fax Configuration Guide, Release 12.2.