What Is VoIP? Voice over IP Explained
The phone call hasn't changed in a century — you still pick up, dial, and talk. What changed is everything underneath it. Your voice is no longer a current on a copper wire owned by the phone company. It's data on your network, and that one shift decides who controls your calls.
VoIP (Voice over Internet Protocol) carries voice calls as digital packets over an IP network instead of the traditional circuit-switched telephone line. It turns a phone call into software — which is what makes it cheap, flexible, and something you can run yourself.
How VoIP actually works
When you speak into a VoIP phone, your voice is sampled, compressed by a codec, and broken into small packets. Those packets travel across the network to the other party, where they're reassembled and played back — all within a few tens of milliseconds, fast enough that it feels like a normal call.
Two things happen on every call: signalling (setting the call up, ringing, hanging up) and media (the actual audio). They're handled separately — signalling by a protocol like SIP, media by RTP. Keep that split in mind; nearly every security decision about a phone system comes down to how each of those two streams is protected.
Why businesses moved to VoIP
The old PSTN charged per line and per minute, and adding capacity meant physical wiring. VoIP collapsed all of that into software running on standard servers. One system now carries voice, video, messaging and conferencing; adding an extension is a configuration change, not an electrician's visit.
It also unlocked features that were expensive add-ons on legacy systems — auto-attendants, call queues, voicemail-to-email, call recording, presence — because they're now just software modules.
What VoIP changes about control
Because a VoIP system is software, you get a choice the PSTN never offered: where it runs. It can live in a provider's cloud, or on a server you own, inside your own building or network. For most businesses that's a convenience question. For a bank, a hospital or a defence unit, it's a sovereignty question — because every call now produces data (who called whom, when, recordings) that has to live somewhere.
That's the thread running through this whole glossary: VoIP made communications programmable, and programmable means you can decide to own it rather than rent it.
Frequently asked questions
Is VoIP cheaper than traditional phone lines?
Usually, yes — especially for multi-site and international calling, because calls ride your existing data network and SIP trunks replace per-line rentals. The bigger saving is operational: changes are software, not wiring.
Does VoIP work without the internet?
VoIP needs an IP network, but not necessarily the public internet. A self-hosted VoIP system can run entirely on a private or air-gapped network with no internet at all — which is exactly how sovereign and defence deployments run it.