VoIP (Voice over IP) is a means of encoding, transmitting and re-assembling sound over a network within IP packets. IPT (IP Telephony) is a means of making a Telephone call over an IP network without your user being aware that they are doing so. VoIP takes many forms; most commonly adopted forms are SIP and SCCP, both of which facilitate voice communications across the now IP network.
The underlying technology and capabilities of VoIP services vary dramatically, and like most things, they are separated by cost as well as capabilities.
Business-class VoIP services generally use SIP (Session Initiation Protocol) to establish a prioritized connection to a central PBX which then routes the calls to their destination. The exceptions to the rule are SCCP, H323 etc. These provide more of an IPT service as the end customer just dials the number as always, and does not know nor care whether they are transported over the PSTN (Public Switched Telephone Network), an IP based private connection, or the public internet.
The actual voice data is usually transported across the network using one of two primary codecs (Encoder/Decoder), G.711 and G.729. Both encoding formats have their strengths. G.711 is the same encoding system used on the global telephone network and offers greater quality than a normal analogue call. G.729 is very bandwidth-efficient whilst still offering high audio quality and is often used to provide VoIP services across domestic broadband internet connections.
All forms of VoIP have their benefits, some have become more flexible, others have developed over the last two years at a greater pace, or with greater acceptance and virtually all are delivered to the end user over the same network as their desktop services.