Jitter

Jitter is defined as a variation in the delay of received packets. That is, Jitter = Delay Variation

Due to network congestion, improper queuing or configuration errors, this steady stream can become lumpy, or the delay between each packet can vary instead of remaining constant

Determine the severity of Jitter

Once a call is up and active, jitter is suspected, telnet or ssh to one of the gateways involved

Enter show voice call summary

Selected the port where the jitter is experienced

Enter show voice call [port]

View the ***DSP VOICE VP_ERROR STATISTICS*** section in the output

What causes Jitter?

Jitter is generally caused by congestion in the IP network. The congestion can occur either at the router interfaces or in a provider or carrier network if the circuit has not been provisioned correctly.

Reference

https://www.cisco.com/c/en/us/support/docs/voice/voice-quality/18902-jitter-packet-voice.html#topic1