Sometimes it's not the service itself that matters as much as the condition of that last mile of wire between the feeder cables and your house, or the drop from the tv cable to your house. Good internet speeds do not always indicate good voip speed. That's why I suggested trying skype.
It's free for a lot of services and can give you a feel for voice quality and delay times for your connection. Nothing more annoying than talking to a person with a 0.5-2 second delay in transmission.
If quality is bad for everyone you talk to on skype, it's probably due to your internet connection. If it's just bad for some people, it's due to their connection.