What are the advantages and disadvantages of IP telephony

What are the advantages and disadvantages of IP telephony? The primary advantage is that IP telephony takes advantage of newer technology which is more efficient from a cost and technical perspective. For end users it allows you to run voice services over one shared service (running voice, data and video) versus buying multiple services and circuits from service providers. For service providers it allows them to provide customers multiple services over one core IP network rather than having to build and maintain separate networks for different services.