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.
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.