I think we discovered, and agree with, Rather Be Fishing with regards to network capability being the center of VoIP happiness or VoIP disaster.

We have eight ShoreTel sites, somewhere around 150 users. No trouble with ShoreTel, occasional trouble with the network, which then affects the VoIP quality. (We STILL don't have that MPLS network on line). We'll have another seven or eight ShoreTel sites by March, and hope to have the MPLS on line January-ish. I'll let you know how it goes.