Latency and Jitter: Latency is the time gap that it takes the audio to reach the user. When you have the firewall configured to prioritize VoIP traffic, latency is not an issue. Jitter is a type of poor audio. Jitter is often an Internet Service Provider (ISP) issue but could also be a configuration issue. Jitter is what happens when the voice packets are delayed or queued, preventing them from getting to the destination or they get there delayed. Acceptable jitter is up to 250ms roundtrip.
Traffic Prioritization: Your firewall will need to prioritize traffic for the business needs. When using VoIP, this traffic should be a priority over other traffic. For example, if someone is downloading the employee manual and VoIP is not a priority this download could deteriorate call quality.
Poor Internet Connection: This is where internet speed becomes important. If you do not have the recommended speeds with your internet connection then you could experience packet loss and poor call quality.
Internal Network misconfigured: ShoreTel Phones will need to be able to reach the default gateway of the VPN connection they’re behind. Audio packets will NAT from LAN to WAN with RingLeader SBCs.