Quite simply, jitter is most likely and commonly due to increased latency within a network, which is due to increased congestion. As I mentioned above:

• Network Congestion — probably the most obvious and common cause of jitter is simply an overcrowded network. If you have too many devices looked up to the same network, all being used at the same time, you will run out of bandwidth, and slow your connection to a crawl.

Insufficient bandwidth to handle a VoIP call will lead to packets being dropped or delivered out of order.

• Wireless Networks — while a wire free network enables mobility and frees us from cables running through the office, chances are you will be experiencing a degraded network connection. While fine for our mobile devices, WiFi isn’t necessarily powerful or stable enough to rely on for our phone calls.
• Bad Hardware — our internet networks are generally made up of a couple of different pieces of hardware, at least a modem and a router, sometimes switches as well. Bad hardware, like an outdated modem, a damaged Ethernet cable, or a misconfigured router can lead to call quality issues.

According to Cisco again, this congestion could “occur either at the router interfaces or in a provider or carrier network.” Sadly, in the case of interference within a provider or carrier network, things are out of your hands. But we’ll focus on what we can change, and touch a bit more on how to identify and even correct latency, and therefore ultimately fixing jitter.