Core Issue
PSTN callers experience robotic or metallic voice when calls are made to the IP phone behind the 28xx series voice gateways. When the problem does occur, it happens on all the calls through the gateway.
Resolution
This issue is documented in the Cisco bug ID CSCsd90851. In order to resolve this issue, use the test voice driver, option 2, or option 1 in order to reset DSPs, or reload the router. Also it is recommended to replace PVDM or upgrade Cisco IOS on the gateway.
Refer to document In Cisco CallManager 4.x with CMM, if a conference call is established between the IP phones and the PSTN phone through the MGCP, there is only one-way audio from the PSTN to the IP phones and no audio between the IP phones.
Refer to these documents for more information about the voice quality and troubleshooting voice quality issue:
