Skype and Vonage are both hosted services meaning that they maintain the equipment. They use the open Internet to route calls, and in some instances may not be acceptable if you do not have enough bandwidth in your Internet connection. As for the others, there are gateways that help with all, but in general, I feel that it is much easier to administrate if you have one or the other at both ends. The advantages of having your own system is that you can set up VPN access and even use dedicated Internet lines if you chose.
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