I came across this nice article today by Brett Markham, "Not all VoIP is created equal". I believe it could help VoIP users understand why their quality of service could be lacking.

"Every week BELD Broadband’s help desk receives calls from broadband Internet subscribers who use third-party Voice over Internet Protocol (VoIP) services such as Vonage and SunRocket. These calls are usually complaints about call quality. The reason we receive such calls explains both the strength and weakness of these services."

"VoIP is a protocol that handles dialing functions and translates analog voice signals into packets of data similar to music mp3s, though of lower fidelity. The packets of data are then routed across one or more networks in order to terminate in a telephone switch that connects the caller with the phone number being dialed."