Latency is the length of time it takes for your words to be received by a listener at the other end of a phone connection, typically milliseconds. According to a white paper from Brooktrout Technology, latency starts to affect phone conversations when it exceeds 150 milliseconds each way.
It's unacceptable when it exceeds 450 milliseconds (nearly half a second). The company recommends engineering a VoIP system so that latency is always below 200 milliseconds.
When different packets reach the receiver at different times, the out-of-order packets reproduce as gobbledygook called jitter. If you suffer a jitter problem, consider the network big picture. If there is a correlation between jitter and bandwidth usage (say during a full scale network backup or large data transfer), the problem may well be overall network usage. If there is no direct correlation, then jitter may be coming from congestion on the Internet, outside your LAN.
Packet loss happens when some packets never reach their destination for whatever reason. The problem can cause completely dropped calls. Related conditions include dropped packets (too much data arrives at the receiving server too quickly) and packet delay (data takes the long way around the Internet).
Engineers can beat packet loss by separating voice and data traffic (using a virtual LAN or vLAN, a technique that creates independent logical networks within a physical network). Full duplex, non-blocking switches can help to avoid collision and packet loss as well.
Intelligent routing protocols, like MPLS (multi-protocol label switching) and OSPF (open shortest path first), prioritize network traffic. These protocols intelligently optimize the routing of network traffic in accordance with predetermined parameters.
In addition, know that there are network analysis tools that can identify and eliminate congestion points on the network. According to a VoIP network best practices list provided by Network Instruments ®, metrics to assess VoIP call quality include jitter, MOS, R-Factor, gap density, burst density, quality of service prioritization, and compression techniques. See? You do need to call in a professional.