You are connecting your voice gateway to the PSTN through digital lines. What is the command you should use for clocking synchronization?

The 'clock source line' command instructs the router to derive its clocking signal from the incoming digital line provided by the PSTN carrier. This is required because the PSTN operates as the network clock master, and the gateway must slave to it to avoid slips and voice quality degradation. Clocking mismatches on digital circuits cause bit errors and audible artifacts.

'clock source internal' configures the router to use its own internal oscillator as the clock master, which is used when the router itself must be the clock source, not when slaving to the PSTN.

'clock source free-running' is not a standard Cisco IOS command for T1/E1 interface clock synchronization in this context.

'clock source external' references an external hardware clock input, not the incoming line signal from the PSTN carrier.