Incorrect clock synchronisation on the SCN side of the Cisco VG30D Voice Gateway can cause severe problems with modem and fax quality, and lesser problems with voice quality. Calls may also get dropped.
Ensure the clock is set by using the Gateway Management Interface. Go to Configuration / Gateway / Set PCM Clock Source. Select whichever port can provide a network-provided clock. It will synchronise to the source that is selected if a valid clock is present or synchronise to the other port if a clock is not present. If you are unable to synchronise to either port, the Cisco VG30D Voice Gateway internal source will get used.
The Cisco VG30D Voice Gateway does not get specified to have its own very high resolution clock tied to the National Public Network frequency, nor does it get specified to accept a Kilostream clock input.