VOIP-What is the best tool avialible for tracking problems on the network?

We just implemented a VoIP system at my place. We went with a Mitel 3300 in 4 different locations. My problem is I sometimes get JitterBuffer Underun and it is enough to make a big impact on the current call. Because we are new to this, I am not sure what product is out there to dentify any anomolies in the network that can cause these disruptions. I am look for any Ideas or if anyone can share thier problems and resolutions.

Thanks