Voice over Internet Protocol (VoIP) is a general term for a family of transmission technologies for delivery of voice communications over IP networks such as the Internet or other packet-switched networks. Other terms frequently encountered and synonymous with VoIP are IP telephony, Internet telephony, voice over broadband (VoBB), broadband telephony, and broadband phone.

VoIP systems usually interface with the traditional public switched telephone network (PSTN) to allow for transparent phone communications worldwide.

VoIP systems employ session control protocols to control the set-up and tear-down of calls as well as audio codecs which encode speech allowing transmission over an IP network as digital audio via an audio stream. Codec use is varied between different implementations of VoIP (and often a range of codecs are used); some implementations rely on narrowband and compressed speech, while others support high fidelity stereo codecs.

Corporate Use

Because of the bandwidth efficiency and low costs that VoIP technology can provide, businesses are gradually beginning to migrate from traditional copper-wire telephone systems to VoIP systems to reduce their monthly phone costs.

VoIP solutions aimed at businesses have evolved into "unified communications" services that treat all communications—phone calls, faxes, voice mail, e-mail, Web conferences and more—as discrete units that can all be delivered via any means and to any handset, including cellphones. Two kinds of competitors are competing in this space: one set is focused on VoIP for medium to large enterprises, while another is targeting the small-to-medium business (SMB) market.

VoIP also offers the advantage of running both voice and data communications over a single network which can represent a significant saving in infrastructure costs.

Other advantages that appeal to business is that the per extension prices of VoIP are lower than those of PBXs or key systems. Also, VoIP switches rely on commodity hardware, such as PCs or Linux systems, so they are easy to configure and troubleshoot. Rather than closed architectures, these devices rely on standard interfaces.

VoIP devices also have simple, intuitive user interfaces, so employees can often make simple system configuration changes. Features such as dual-mode cellphones enable users to continue their conversations as they move from an outside cellular service to an internal wi-fi network. The bundling means employees no longer have to carry a desktop phone and a cellphone, so companies can reduce their telecommunications equipment costs. Maintenance also becomes simpler, because there are fewer devices to oversee.