Most telephone calls are still delivered over the Public Switched Telephone Network (PSTN). This involves a network of Central Offices (COs) which are connected via trunk lines and the "last mile" to your phone is delivered via a dedicated line. This is quite an inefficient system because even when the phone is not being used the bandwidth of the line is still tied up.
In the case of IP (Internet Protocol) Telephony, voice signals are encapsulated in data packets and transported over private or public data networks such as the Internet. Because these voice signals are now transmitted in packets in the same way as email, document files or web pages might be transmitted, there is no need for a separate voice network to handle calls.
IP telephony enables the convergence of networks. You no longer need separate voice and data networks. You can now handle voice, data and even video on a single network. This reduces costs for wiring, support staff and service contracts.