How VoIP works, step by step
A traditional landline holds open a dedicated circuit between two phones for the length of a call. VoIP works differently: it is packet-switched, so your voice travels as many small packets of data that are reassembled at the other end.
- Digitize: a microphone captures your voice and an analog-to-digital converter samples it into digital values.
- Compress: a codec encodes the audio, often compressing it to save bandwidth.
- Packetize: the encoded audio is split into small packets, each labeled with its destination.
- Transmit: the packets cross your network and the internet. They may take different routes.
- Reassemble: the receiving device puts the packets back in order, decodes them, and plays the audio.
The protocols behind a call: SIP and RTP
Two protocols do most of the work. SIP (Session Initiation Protocol) handles signaling: it finds the person you are calling, makes their phone ring, negotiates the call settings, and ends the call when you hang up. RTP (Real-time Transport Protocol) carries the actual voice packets while the call is in progress. In simple terms, SIP is the phone ringing and RTP is the conversation.
When a VoIP call has to reach someone on a regular phone line, a media gateway or SIP trunk provider translates between the IP network and the public switched telephone network (PSTN).
Codecs and bandwidth
A codec decides how voice is compressed, which affects both sound quality and how much bandwidth a call uses. Three are common in business calling:
- G.711: uncompressed, broadcast-quality voice at 64 kbps of audio per direction. With network overhead a call typically uses around 80 to 90 kbps.
- G.729: heavily compressed at 8 kbps of audio per direction, roughly 30 to 35 kbps with overhead. Uses less bandwidth, with somewhat lower audio quality.
- Opus: a modern, adaptive codec that scales from low bitrates up to high-fidelity audio depending on network conditions.
What you need to use VoIP
You need an internet connection that is stable more than it is fast, a VoIP service provider, and a device to make calls. That device can be a softphone app on a computer or smartphone, an IP desk phone, or an analog phone connected through an adapter. Business VoIP is usually delivered as a cloud phone system, where the provider hosts the call-handling software so there is nothing to install on site.
VoIP vs landline
- Cost: VoIP usually avoids per-line hardware costs and makes long-distance and international calling cheaper.
- Flexibility: your number and extension follow you to any device with an internet connection.
- Features: voicemail-to-email, call queues, auto attendants, call recording, and analytics are typically built in.
- Dependencies: VoIP relies on your internet connection and on power for your network equipment, so reliability planning matters. See VoIP call quality for how to protect it.
Common questions
- What does VoIP stand for?
- VoIP stands for Voice over Internet Protocol, sometimes written Voice over IP.
- Do I need special equipment for VoIP?
- Not necessarily. A computer or smartphone with a softphone app and a headset is enough. You can also use IP desk phones, or analog phones with an adapter.
- Is VoIP the same as a cloud PBX?
- No. VoIP is the underlying technology for carrying voice over IP. A cloud PBX is a complete business phone system, hosted by a provider, that uses VoIP and adds routing, voicemail, extensions, and other features.
- Does VoIP work during an internet outage?
- Not on its own. Many businesses configure failover so calls forward to mobile numbers or switch to a backup connection if the primary internet link fails.
- Can I keep my existing phone number with VoIP?
- In most cases yes, through number porting. Timelines and requirements depend on your current carrier and country.