Direct answer
Every claim is sourced below
VoIP, short for voice over Internet Protocol, is the group of technologies that carries a voice call as data over an IP network. The term describes how a call travels. It doesn't describe a product, so two services sold as VoIP can have little in common beyond that.
The NIST glossary defines VoIP as a group of technologies for voice, video and collaboration communications over a data network that uses internet protocols. The definition is broad, and the breadth explains a lot about how the word gets used.
A data network that uses internet protocols is a wider thing than the internet. Internet Protocol is a set of rules for addressing and moving data. The same rules run on an office network and on a private connection between a business and its provider. So a call can be VoIP without touching the public internet, and we cover that in whether VoIP requires an internet connection.
The definition also names no particular standard. The Session Initiation Protocol, or SIP, is one published standard for setting up calls, and a service can be VoIP without using it.
Video and collaboration sit inside the same definition as voice. That's why the line between a VoIP service and a wider communications package is hard to draw from the label. We compare the two in the difference between UCaaS and VoIP.
Suppose you call a colleague from an app on your laptop. The microphone picks up your voice, and the app converts the sound into a stream of numbers. It sends that stream as a series of small packets, each addressed to the other end. Your colleague's device puts the packets back in order and plays them as sound. The same thing happens in the other direction at the same time.
Two published standards show how the work is divided. The Real-time Transport Protocol, or RTP, carries the audio. It gives each packet a sequence number and a timestamp so the receiver can rebuild the order. Its specification also says what RTP leaves to the network underneath. RTP doesn't guarantee that a packet arrives, or that it arrives on time. So the network a call crosses affects how it sounds, along with the devices at each end and the way the audio is encoded.
SIP handles the call around the audio. It locates the other party and negotiates the session, then closes the session when someone hangs up. The setup messages and the audio can travel by different routes.
We follow a call the whole way in how VoIP works, and from the handset's side in how a VoIP phone works.
Some of the forms VoIP is sold in
| Form | What you subscribe to | Where the phone system runs |
|---|---|---|
| Hosted phone system | Users on a provider's phone system, often priced per user per month | In the provider's cloud |
| SIP trunking | A connection between a phone system you run and the public telephone network | On equipment you run |
| Calling inside a collaboration app | Calling as one feature of an app such as Microsoft Teams | In the app provider's cloud |
| Digital landline | Home or office phone service delivered over broadband | In the provider's network |
A single provider can sell more than one of these forms. Atlantech Online does. Its UCaaS service is a hosted phone system that Atlantech says runs on its cloud, so the office doesn't keep one on site. Its pricing cards start at $12 per user per month for the Business Line and $18 for the Premium Line, and the softphone app is listed on the Premium card. Its business phone service page describes SIP trunking too. A business that has its own phone system can have service delivered to that system over a SIP connection.
The hosted system and the SIP connection are both VoIP. A buyer of the first gets a phone system. A buyer of the second gets a connection for a phone system they already own. We explain that second purchase in what SIP trunking is and in the difference between SIP trunking and VoIP.
Microsoft Teams shows calling inside a collaboration app, and a limit that comes with it. Microsoft's Teams Phone overview says that by default, calls between Teams users in one organization are handled within Teams and don't go to the public telephone network. Calling an outside number takes a Teams Phone license and a separate connection to that network. So a service can carry voice over IP and still not reach an ordinary phone number until that connection is added.
The digital landline is the reason "VoIP or landline" is less clear-cut than it sounds. Ofcom's guide to digital landlines describes UK home phone service that runs over broadband, with the phone plugged into the router. That service is still sold as a landline, and it's carried as VoIP. We go through the comparison in how VoIP is different from a landline.
Because VoIP names a method, calling a service VoIP leaves the buying questions unanswered. The word doesn't say whether the service reaches ordinary phone numbers or which devices it supports. It doesn't say what the plan includes, or how calls will sound on your network. Those answers come from the subscription and from the path the calls take.
So when a quote says VoIP, find out which form it is before comparing prices. A per-user price for a hosted phone system and a price for a trunk into your own system pay for different things. Once you know the form, the practical list is short, and we lay it out in what you need to use VoIP.
Legal Information Institute, Cornell Law School
“Enables real-time, two-way voice communications; requires a broadband connection from the user's location; requires internet protocol-compatible customer premises equipment; and permits users generally to receive calls that originate on the public switched telephone network and to terminate calls to the public switched telephone network.”
AT&T Business
“VoIP is the technology that converts your voice into a digital signal, allowing you to make a call directly from a computer, a VoIP phone, or other data-driven devices.”
IETF
“an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls.”
IETF
“RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data.”
International Telecommunication Union (ITU-T)
“Pulse code modulation (PCM) of voice frequencies. Status: In force.”
IETF
“It scales from low bitrate narrowband speech at 6 kbit/s to very high quality stereo music at 510 kbit/s.”
TelcoBridges
“G.711: around 85 to 87 kbps per direction. G.729: around 31 kbps per direction.”
Legal Information Institute, Cornell Law School
“specifically advise every subscriber, both new and existing, prominently and in plain language, of the circumstances under which E911 service may not be available.”
Wiley Rein LLP
“Fixed VoIP services: starting January 6, 2021. Non-fixed VoIP services: starting January 6, 2022.”
IETF
“This document defines a mechanism for securely identifying originators of SIP requests.”
Davis Wright Tremaine LLP
“all originating and terminating voice service providers to implement the STIR/SHAKEN caller ID authentication framework in all IP portions of their networks by June 30, 2021.”
Atlantech Online
“Switching to a VoIP system saves small businesses up to 45 percent on their monthly phone costs.”