The question is... which is right for your business. Hosted Voip has matured and is a viable, feature rich product. Managed on-site can be a good choice depending on your needs. On premise gives you a bit more piece of mind, but it comes with maintenance costs.
You will need to do some homework. First, assess your needs and gather all of your current costs. Then compare and contrast the costs and risks of off site telephone service to on premise options.
Let me know if I can help!