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Not always. VoIP requires an IP network path between your phone and the system that handles its calls, and that path can be public or private. A call can cross the internet, run over a private connection to your provider, or stay between two phones inside one building. Which of those your calls use depends on how the service is set up. To call ordinary phone numbers, the call also has to reach a service connected to the public telephone network.
Where a VoIP call can travel
| Path | Does it cross the public internet? | What it depends on |
|---|---|---|
| Hosted phone service reached over your regular internet connection | Yes | Working internet service at your site |
| An access connection on your voice provider's own network | Not between your site and the provider's voice switch | That provider's connection to your building |
| An internal call on a phone system kept on site | No | The local network and the phone system staying up |
The "IP" in VoIP is Internet Protocol, the method networks use to address data and move it. The public internet runs on that protocol. So does the network inside your office, and so does a private link between two sites. A VoIP phone needs a network path to the system that handles its calls and a path for the audio. Any IP network that reaches both can carry the call.
The internet is one such network. The FCC's consumer guide to VoIP describes the technology as converting your voice into a digital signal that can travel over the internet. The federal definition of interconnected VoIP, the kind of service that exchanges calls with the public telephone network, lists a broadband connection from the user's location among its conditions. So an interconnected VoIP service, by that definition, needs a broadband connection at your end. Whether that connection carries the call across the public internet or along a private path is a matter of how the service is set up.
Wi-Fi causes a related mix-up. Wi-Fi connects a phone or laptop to your local network. The internet service that network uses is a separate thing. A softphone on Wi-Fi with no internet behind it can still reach anything that network routes to, such as a phone system on site or a private link to another office. It can't reach a service that's only available over the internet.
How much connection a call needs is its own question. We answer it in how much bandwidth a VoIP call uses, and we compare access types in what type of internet connection you need for VoIP.
Atlantech Online described two ways to connect an office in a 2016 article on connections for VoIP. In the first, a business buys internet from one company and phone service from another. A call leaves the office on the internet connection, crosses the internet to the voice provider's switch, and goes from there to the public telephone network. In the second, the office's connection is on the same network as the voice switch.
Atlantech wrote in 2016 that calls on its network didn't have to travel across the internet to reach the phone service. The article said its voice switch was in its own data center and customer connections were on its network, so the packets went from the office to the switch. A call like that is still VoIP. It travels as IP packets the whole way to the switch. It just never crosses the public internet on that stretch.
What changes is who runs the path. On the public internet, a call crosses networks that neither you nor your voice provider operates. On a connection to the voice provider's own network, one company runs the equipment at both ends and, as the article puts it, can configure that equipment to put voice traffic first.
This arrangement covers the office and no further. Someone working from home on another company's internet service reaches the phone service over that connection. And the business still pays for a data connection to the building, even though the calls don't use the public internet to get to the switch.
Suppose a company runs its phone system on a server in its own office, and two employees call each other by extension. Both phones and the phone system sit on the office network, so that call needs no outside connection. It keeps working if the internet service fails, as long as the local network and the phone system have power.
Outside calls are different. They need a route to the public telephone network, such as a trunk from a carrier. If that trunk rides the internet connection that failed, outside calling stops while internal calls carry on.
Hosted service reverses the picture. The system that connects calls lives at the provider, so two desk phones on the same office network have to reach the provider before one can ring the other. If the only path to the provider is the connection that failed, those phones stop, even for a call across the hall. They keep working when a second connection is up and the network is set up to route them over it. Equipment on site that takes over is another approach.
Microsoft documents the on-site approach for Teams Phone with Direct Routing. Its Survivable Branch Appliance guidance describes a branch that loses internet while its internal network stays up, along with the session border controller that connects the branch to the telephone network. With the appliance in place, the Teams client switches to it, and people can keep making and receiving outside calls.
The conditions are specific. The appliance comes through the session border controller's vendor, and the controller has to be configured for media bypass. The supported clients Microsoft lists are the Teams desktop apps for Windows and macOS and Teams Phones. The desktop apps have to run on a physical machine, and web clients and virtual machines aren't supported. Microsoft says location information isn't shared on emergency calls in this mode. Its scenario assumes users can still reach the session border controller that provides the telephone connection, so that connection can't depend on the internet link that failed.
When the office internet connection is the only path to a hosted service, an outage takes the desk phones offline until the connection returns. A mobile app gives the service a second path. Atlantech's UCaaS service lists a mobile app for Android and iOS for taking business calls anywhere. Suppose an employee has an app like that, and their mobile phone still has working cellular service. The app reaches the hosted phone system over the carrier's data network, which doesn't run through the office. That employee keeps taking business calls while the desk phones wait for the connection.
Power is the other dependency. The FCC's guide warns that some VoIP services don't work during power outages and that the provider may not offer backup power. A desk phone, the network switch it plugs into and the router all need electricity. A call fails if any one of them is dark.
Emergency calling is where these limits carry the most weight. A VoIP phone that can't reach its service can't reach 911 through that service either. We cover what to plan for in whether you can dial 911 from a VoIP phone.
Electronic Code of Federal Regulations (eCFR), Office of the Federal Register
“An interconnected Voice over Internet Protocol (VoIP) service is a service that: (i) Enables real-time, two-way voice communications; (ii) Requires a broadband connection from the user's location; (iii) Requires internet protocol-compatible customer premises equipment (CPE); and (iv) Permits users generally to receive calls that originate on the public switched telephone network and to terminate calls to the public switched telephone network.”
Electronic Code of Federal Regulations (National Archives / GPO)
“Each interconnected VoIP service provider shall specifically advise every subscriber, both new and existing, prominently and in plain language, of the circumstances under which E911 service may not be available... Such circumstances include, but are not limited to, relocation of the end user's IP-compatible CPE, use by the end user of a non-native telephone number, broadband connection failure, loss of electrical power, and delays that may occur in making a dispatchable location available.”
Cybersecurity and Infrastructure Security Agency (US-CERT)
“VoIP requires a connection to the Internet through an ISP, a VoIP service to extend the reach to traditional landlines, and VoIP software to actually place calls. Plain Old Telephone Service (POTS) requires none of these prerequisites... During a typical power outage, VoIP becomes unavailable because VoIP devices (computers, routers, adapters) usually rely on a power source to function... 911 services are not guaranteed with a basic (VoIP to VoIP) setup.”
IETF
“SIP is an agile, general-purpose tool for creating, modifying, and terminating sessions that works independently of underlying transport protocols and without dependency on the type of session that is being established... SIP runs on top of several different transport protocols.”
IETF
“This document describes a method by which a Service Provider may use an IP backbone to provide IP Virtual Private Networks (VPNs) for its customers... Data packets are tunneled through the backbone, so that the core routers do not need to know the VPN routes.”
IETF
“Opus supports all bitrates from 6 kbit/s to 510 kbit/s... For a frame size of 20 ms, these are the bitrate "sweet spots" for Opus in various configurations: o 8-12 kbit/s for NB speech, o 16-20 kbit/s for WB speech, o 28-40 kbit/s for FB speech.”
International Telecommunication Union (ITU-T)
“Although a few applications may be slightly affected by end-to-end (i.e., "mouth-to-ear" in the case of speech) delays of less than 150 ms, if delays can be kept below this figure, most applications, both speech and non-speech, will experience essentially transparent interactivity... While it is recommended that a one-way delay of 400 ms should not be exceeded for general network planning...”
Cisco
“Total packet size = (L2 header: MP or FRF.12 or Ethernet) + (IP/UDP/RTP header) + (voice payload size)... Bandwidth = total packet size * PPS. G.711: 64 Kbps codec bit rate, 160 bytes voice payload, 50 PPS, 87.2 Kbps bandwidth (Ethernet). G.729: 8 Kbps, 20 bytes, 50 PPS, 31.2 Kbps.”
Microsoft Learn
“During an internet outage, the Teams client switches to the SBA automatically, and ongoing calls continue with no interruptions... Making PSTN calls through the local SBA/SBC with media flowing through the SBC... VoIP calls between local users. If both users are registered behind the same SBA, a VoIP call can be initiated... Sharing location information during an emergency call (E911). Users can still make emergency call but location information won't be shared.”
RingCentral (vendor product documentation)
“If your deskphone is offline due to an internet outage, incoming calls can automatically be forwarded to another extension or external number.”
OnSIP
“Instead of copper wiring, phones connect to the PBX server over your office's Local Area Network (LAN)... Free internal calling: Employees can call each other's extensions without consuming outside lines or incurring charges.”
Nextiva
“VoIP typically requires 80-100 kbps of bandwidth per concurrent call... Codec / 10-Minute Call / 1-Hour Call. G.711 102 MB 613 MB. G.729 36 MB 219 MB.”