VoIP, better known as Voice over Internet Protocol is a protocol optimised for the transmission of voice through the internet or other packet switched networks. VoIP is often used abstractly to refer to the actual transmission of voice. It is most commonly known as IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband.

The companies that generally provide VoIP services are commonly referred to as providers, and protocols which are used to carry voice signals over the IP network are commonly referred to as Voice over IP or VoIP protocols. Voice over IP protocols carry telephony signals in the shape of digital audio, typically reduced in data rate using speech data compression techniques, encapsulated in a data packet stream over IP. There exist two types of PSTN to VoIP services. Direct Inward Dialing (DID) and access numbers. DID supports in connecting the caller directly to the VoIP user while access numbers need the caller to input the extension number of the VoIP user.

VoIP has succeeded in grabbing the attention with much facilitated tasks and by providing services that may be more difficult to implement or expensive using the more traditional PSTN. Conventional phones are connected directly to telephone company phone lines and are kept functioning with the help of back-up generators or batteries located at the telephone exchange that in the event of power failures. Generally, household VoIP hardware uses broadband modems and other equipment powered by household electricity that could be subject to interruptions in the absence of an uninterruptible power supply or generator.

The support of sending faxes over VoIP is still limited. The existing voice codecs are not designed in a suitable way for fax transmission. The efforts to solve this are underway by defining an alternate IP-based solution for delivering Fax-over-IP, namely the T.38 protocol. Another possible solution to overcome the drawback is to treat the fax system as a message switching system which does not need real time data transmission - such as sending a fax as an email attachment or a remote printout.