Conventional telephony is a switching circuit based communication system in which two persons are physically connected for the process of connection to take place.
There exists a dedicated path for flow of signal from one person to another person and vice-versa.
There is real time data flow occurs between the persons connected As a result there is no lag in the voice from one person reaching the other person. Whereas, Internet is a packet based communication system and here data is sent in the form of packets, which are then routed over the internet to the destination. The path, the packet follow to their destination depends on the traffic at any instant in any path given.
It is also not necessary that, the entire packet will follow the same path to the destination. Also real time voice data pose a complex problem when travelling over the Internet. Whenever, data packets face a bottleneck in the process of transmission, the communication takes place in jerks or delays. Sometimes, delays cause message to be incoherent. Although, the present scenario of Internet Telephony is not rosy and the trend of using these products is growing.