I have seen so many complaints online about problesm with different VoIP providers (Vonage, Packet8, SunRocket, etc.). Here are the two most common complaints I see:
The sound quality sucks
Do you have enough bandwidth? I don’t mean enough advertised bandwidth, I mean actual bandwidth. Test it out online on at least 3 different sites. From my experience, you must have at least 128k both ways (up and downstream) in order to get good quality sound.
Many providers give you a choice in how much bandwidth is used. Vonage gives you the choice of 30, 50 or 90k. 30k is slightly lower than landline quality but it still sounds decent. Try turning down the amount of bandwidth used for the calls. I found that quality on my service (Vonage) is excellent at 50k with no difference if I up it to 90k.
My call sound skips or drops but the person on the other end sounds fine
VoIP traffic is just like any other traffic on the network. If you’re surfing the Internet, the traffic has to wait in line like any other request to exit your home network to the Internet. Try getting a good home router with QoS. Do not get the Linksys RT042, it is a horrid router (search Google, you’ll see).
I use the Linksys BEFSX41, with the latest firmware (it added QoS) and it works great. Before I set QoS, my calls would have dropped or delayed sound. I went into the QoS settings and set the PAP2 Vonage device, by MAC address, to high priority. All other service ports are set to medium except the port used by uTorrent is set to low. The sound no longer drops off because the packets for the VoIP are sent out the router first.
Customer service is of no help
Well, that’s a reason to drop the service. Customer service should always be helpful but there are times where they can’t help. Check on using a different provider.
Remember, QoS only affects traffic leaving your home network. If the voice of the person calling you is the one that is dropping, it is a service provider or ISP problem. QoS will not help you in that situation.
Be sure to use an uninterruptible power supply (UPS or battery backup) in case the power goes out. The modem (cable or DSL), router and the VoIP device should all connect to it so that they still work, at least as long as the battery holds up. Some routers are specifically made for use with VoIP providers so they will have phone jacks built into them. That’s just one less device to draw energy from the UPS.
Envy is the art of counting the other fellow’s blessings instead of your own. - Harold Coffin
23.Jun.06 
Great article… I’ve been using Vonage for 2 or 3 months now and I’ll be looking into the QoS stuff you were talking about.