International Calling: Every Term Explained in One Sentence

Telephony vocabulary is unusually opaque, and most of it appears on bills, support pages and price lists without ever being defined. This page defines each term in one sentence, then gives the example that makes it stick.

Dialling a number

Country calling code

The one-to-three-digit code that identifies a country in the international numbering plan. +1 is the United States and Canada, +44 the United Kingdom, +91 India, +7 Russia and Kazakhstan. Codes are assigned by the International Telecommunication Union and are not portable between countries.

Exit code

The digits you dial to tell your own network that the call is leaving the country — replaced by + on any mobile phone. 00 in most of the world, 011 in the United States and Canada, 010 in Japan. The exit code belongs to where you are calling from, never to the number you are calling.

Trunk prefix

A leading digit dialled before a number inside its own country, and removed when the number is written internationally. 0 in the UK, Germany, India and Australia; 8 in Russia and Kazakhstan. Leaving it in is the single most common reason an international call fails.

Area code

The part of a national number that identifies a geographic region, for fixed lines. 20 for London, 212 for Manhattan, 22 for Mumbai. Mobile numbers do not have one; the equivalent leading digits identify the service type instead.

E.164

The international standard for writing a phone number: a plus sign, the country code, and the national number with the trunk prefix removed, up to 15 digits with no spaces. +442071838750. Covered in full in E.164 format.

Subscriber number

The part of the number that identifies one line within an area or a service range — everything after the country code and the area code.

DID (direct inward dialling)

An external phone number that rings one specific phone inside an organisation, bypassing the switchboard. A company with 200 staff may hold 200 DIDs so each person is reachable directly instead of through an extension.

Extension

An internal number behind a switchboard, dialled as tones after the call connects rather than as part of the phone number. See dialling extensions.

Toll-free number

A number where the called party pays for the call instead of the caller — 800, 888 and similar ranges in North America, 0800 in the UK. Toll-free numbers are usually not reachable from outside their own country, and where they are, the call is not free.

Premium rate number

A number charged above the standard rate, with part of the revenue going to the number's owner. 0900 ranges in much of Europe, 1-900 historically in the US. Calls to these are frequently blocked by default on international services.

What a call costs

Termination rate

The fee the receiving network charges to deliver a call onto its own lines. This is the largest component of what an international call costs and the reason the price varies by a factor of fifty between countries — see why calling one country costs 50x more.

Per-second billing

Charging for the exact duration of the call. A 47-second call costs 47 seconds.

Per-minute billing (rounding)

Charging in whole-minute increments, rounding up. A 47-second call costs 60 seconds; a 61-second call costs 120. On short calls this is a 20–30% surcharge that does not appear in the advertised rate.

Connection fee

A flat charge applied when the call connects, independent of duration. On a two-minute call a 50-cent connection fee can exceed the entire per-minute cost.

Interconnection

The commercial and technical arrangement under which two networks exchange traffic. Every international call crosses at least one.

Bill and keep

An interconnection arrangement where networks exchange traffic without paying each other per minute, on the basis that volumes roughly balance. It is why domestic calls in North America cost carriers almost nothing, and part of why calls there are cheap.

Grey route

An unofficial path for international traffic that avoids the official termination fee, typically through SIM boxes in the destination country. Cheaper, illegal in many jurisdictions, and recognisable by stripped caller ID and poor call setup.

Roaming

Using your home mobile subscription on a foreign network. Voice roaming charges are the most expensive way to make an international call in almost every case.

How the call is carried

VoIP (voice over internet protocol)

Carrying a voice call as data over an internet connection instead of over a dedicated telephone circuit. The term covers everything from an app on a phone to a corporate phone system.

SIP (session initiation protocol)

The signalling protocol that sets up, modifies and ends a VoIP call. SIP negotiates the call; it does not carry the audio.

RTP (real-time transport protocol)

The protocol that carries the actual audio once SIP has set the call up. When a call connects but has no sound, SIP succeeded and RTP did not — the classic one-way audio fault.

Codec

The algorithm that compresses voice for transmission. G.711 is uncompressed telephone quality at about 64 kbps; Opus is a modern variable-rate codec typically using 16–24 kbps with better audio. See how much data voice calls use.

DTMF

Dual-tone multi-frequency — the tones a keypad generates, each a pair of frequencies. It is how extensions and menu choices are sent after a call connects.

PSTN

The public switched telephone network: the global circuit-switched telephone system that landlines and mobile voice ultimately connect to. A call "to the PSTN" means a call to an ordinary phone number, as opposed to an app-to-app call.

VoLTE

Voice over LTE — carrying carrier voice calls as data over the 4G network rather than falling back to older circuits. It is what makes calls connect faster and sound wider on a modern mobile network.

Wi-Fi calling

A carrier feature that routes your normal mobile calls over a Wi-Fi network when cellular signal is poor. It uses the Wi-Fi network's data but still consumes your plan's minutes and its international rates.

NAT (network address translation)

The router function that lets many devices share one public internet address. It is the usual cause of one-way audio on VoIP calls, because it can leave the return audio stream with nowhere to go.

Jitter

Variation in the arrival time of audio packets. Beyond what the receiving device's buffer can absorb, it is heard as audio chopping in and out.

Packet loss

Audio packets that never arrive. Above about 3% it produces the robotic, metallic sound people describe as underwater. Diagnosed in call quality troubleshooting.

Latency

The one-way delay between speaking and being heard. ITU-T Recommendation G.114 puts 150 milliseconds as the limit for transparent conversation and 400 milliseconds as the point at which it becomes unacceptable.

Numbers and identity

Number portability

The right to keep a phone number when changing operator, which severed the link between a number's prefix and the network serving it. See number portability explained.

Caller ID (CLI)

The number presented to the person receiving a call. It is asserted by the originating network and not verified end to end, which is why it is frequently wrong or missing on international calls.

STIR/SHAKEN

Standards that let an originating network cryptographically sign the caller ID it asserts, required in the United States from June 2021 and Canada from November 2021. It does not extend across international borders — see caller ID on international calls.

Virtual number

A phone number not tied to a physical line or SIM, which forwards calls to wherever its owner chooses. Used to present a local presence in a country where you do not live.

HLR lookup

A query against a mobile network's home location register to establish whether a number is live and which network currently serves it. It is the only reliable way to determine a ported number's operator, and it is a paid, live query rather than something any offline tool can do.

Billing and account terms

Credit

A prepaid balance consumed at each destination's per-minute rate, as opposed to a pool of minutes. A $5 credit buys 250 minutes to Canada or 7 minutes to a German mobile, because it is money rather than time.

Minute bundle

A fixed allowance of minutes to a defined set of destinations. Simpler to understand than credit and usually worse value, because unused minutes expire and the destination list rarely matches who you actually call.

Expiry

A date after which unused credit or minutes are forfeited. It is the mechanism that makes an advertised rate misleading, since credit you never spend was never a discount.

Fair use policy

A clause limiting an "unlimited" plan, usually by monthly hours or by destination. Worth reading before relying on the word unlimited, because the limit is where the actual product is defined.

Post-paid and pre-paid

Post-paid bills you after the fact against a credit relationship; pre-paid deducts from a balance you funded in advance. Pre-paid cannot produce a surprise bill, which is its main advantage for international calling.

Retail rate and wholesale rate

The wholesale rate is what a provider pays a carrier to deliver a call; the retail rate is what you pay the provider. The gap between them is the provider's margin, and on cheap destinations it is a fraction of a cent.

Bottom line

Most of the confusion in international calling comes from four terms that are routinely mixed up. The exit code belongs to the country you are calling from; the country code belongs to the country you are calling. The trunk prefix is a domestic dialling instruction and must be removed before a number goes international. And the termination rate — a fee charged by a network you will never deal with — is what actually decides the price of your call, not the app you place it with.

Get those four right and the rest of the vocabulary is descriptive detail.