Jitter is the variation in the time between packets arriving, caused by network congestion, timing drift, or route changes. In the context of VoIP, jitter is especially problematic because it affects the steady stream of packet delivery needed for coherent voice communication. When packets are received out of order, the audio can become jumbled and distorted, as the packets don't arrive in the sequence needed for correct playback. Jitter is the specific network metric responsible for causing the symptoms described in the scenario, whereas bandwidth, latency, and delay could contribute to network performance issues but would not specifically cause packets to be received out of order.