VoIP (voice over IP)

VoIP, or voice over IP, is a way of carrying phone calls as data over the internet or any other network that uses the Internet Protocol (IP), instead of over the old telephone network. A call on a messaging app is VoIP. So is a call through an office's cloud phone system. Calls on India's 4G and 5G mobile networks also use the same method. On 4G it is called VoLTE, voice over LTE. On 5G, a call uses either VoLTE or the newer VoNR, voice over new radio.

Here is how it works in general. The microphone's sound is turned into numbers many thousands of times a second. Software called a codec compresses these numbers and packs them into small packets, usually one every 20 milliseconds. Before the voice starts, a set of rules called SIP, the Session Initiation Protocol, sets up the call: it rings the other side and agrees which codec both sides will use. The voice packets then travel over the network alongside all other data. The receiver holds a few packets in a short queue, to even out small differences in when they arrive, and plays them in order.

A call needs little bandwidth, often less than 100 kilobits per second. What it needs is low delay and few lost packets. If packets arrive late or not at all, the voice breaks up or the two people talk over each other. To reach an ordinary phone number, the call passes through a gateway into an operator's network. A session border controller sits at that edge and checks every call that crosses it.

Take, for example, an office employee who answers a call to the company's main number on a laptop. The softphone on the laptop rings, because the cloud phone system sends the call there over the internet.

Consider a support team whose agents work from home in different towns. Contact centre software sends each customer's call to a free agent over VoIP, and records it.

Say a delivery app wants a rider and a customer to talk without seeing each other's numbers. Number masking connects both of them to a temporary number and joins the two calls.

How is VoIP different from an old telephone call? An old call reserved a fixed line through the exchanges for the whole call, even during silence. A VoIP call shares the network with other traffic and sends packets only when needed. That makes calls cheaper, but their quality depends on the network's delay. In India, connecting internet calls to ordinary phone numbers is controlled by telecom licence rules, so businesses connect through a licensed operator.