Check out VoIP Spear (www.voipspear.com). It's a web service that you can use to monitor your VoIP quality. It tracks latency, packet loss, and jitter and calculates a MOS score from 1 (very poor) to 5 (excellent). You can set up VoIP Spear to send you email alerts when the VoIP quality drops too low.

VoIP Spear is free to use for personal use.