What do internet telephony,packet telephony,IP telephony and converged network means?

The first thing all mean the same thing. Which is using IP (Internet protocol) for voice services. Some voice networks are only packet-switched and have no access outside of their own VoIP network. Most VoIP networks have a Gateway that connects to a circuit-switched external network which gives them acces to external calling. One of the gateways responsibilites is to convert G.711 Circuit-switched media (typically a T1 provided by a telco company) to the 7.723 Packet-switched media that will traverse the companies VoIP network. A device called a gatekeeper will then convert the IP address (used by H.323 protocol) to a standard telephone number (E.164 address) that can be used for external calling. A converged network is a network that passes both Voice and Data over the same set of devices. Converged networks generally implement QoS (Quality of service) on all actived network devices to ensure the VoIP has priority over standard data because of it's more rigid demands.