Telecom Acronym Reference: A Plain-English Glossary
· Callr
Telecom is one of those fields where the core ideas are simple but the vocabulary gets in the way. The moment you start building anything with voice or SMS, you run into a wall of acronyms — and most explanations either skip the detail you need or bury you in it.
This reference aims for the middle ground. It covers the terms you genuinely encounter when working with phone numbers, calls, messaging and VoIP, with enough depth to be useful and no more. Use it as a glossary you can come back to.
Common acronyms
These are the terms that show up across almost every telecom and messaging project.
| Acronym | Stands for | What it means |
|---|---|---|
| API | Application Programming Interface | If user interfaces are how people use computers, APIs are how computers use each other. An API defines a standard way for one program to talk to another without needing to know how it works inside. Callr exposes a REST API and webhooks so you can build voice and SMS interactions programmatically. |
| CDR | Call Detail Record | The data produced for every call: start time, duration, hangup time and hangup cause, among other attributes. Analyzed in bulk, CDRs reveal usage patterns and are invaluable for diagnosing and debugging your call flows. |
| CLI / CID / CLID / CLIR | Caller Line Identification / Caller ID | Identifies the number a call originates from. It can be presented or withheld (CLIR is the restriction of that presentation). The SMS equivalent is the Sender ID, which can be alphanumeric or numeric depending on whether replies are needed and on the rules in each country. |
| DID | Direct Inward Dialing number | Originally a way for a company to assign a range of numbers to individuals without a dedicated PBX line each. Today it simply means a phone number you can buy or port. |
| DLR | Delivery Receipt / Report | The status a mobile network returns to the sender of an SMS, with its timeline — created, sent, received and updated dates. Webhooks let you react to each DLR the moment it arrives instead of polling for it. |
| DTMF | Dual-Tone Multi-Frequency | The system behind the tones you hear when pressing keys on a phone. Each key sends a unique combination of a row tone and a column tone. It replaced rotary dialing and is what powers IVR menus, where each key is bound to a different action. |
| E.164 | International numbering plan | The international standard for phone numbers: a +, a country code of one to three digits, then up to fifteen digits in total for the subscriber number. |
| IVR / VRU | Interactive Voice Response / Voice Response Unit | Technology that lets a caller interact with a computer over the phone using key presses (DTMF) and speech. It’s used for purchases, lookups, banking, support and call prequalification, on both inbound and outbound calls — though inbound is by far the most common. |
| LVN | Long Virtual Number | A standard-length number (for example +44 9865 80044) used to receive both SMS and calls. Compared with country-specific, sometimes-shared shortcodes, an LVN is available internationally and dedicated to you. |
| MO / MT | Mobile Originated / Mobile Terminated SMS | MO messages travel from a customer’s handset into your system. MT messages travel the other way — from your system out to a customer or prospect’s mobile. |
| PSTN | Public Switched Telephone Network | The combined telephone networks run by local, regional and national operators — the traditional infrastructure that carries calls. Shared standards let these networks interconnect seamlessly across countries. |
| QoS | Quality of Service | The overall performance of a telephony network as experienced by the people using it. It’s judged on response time, signal-to-noise ratio, echo, crosstalk and dropouts, and it matters most for VoIP and SIP. |
| VoIP | Voice over Internet Protocol | A set of technologies for carrying voice and multimedia over the internet instead of the PSTN. It relies on protocols such as SIP and RTP, and underpins most software calling today. |
VoIP-specific acronyms
Once you go deeper into VoIP, a second layer of vocabulary appears — mostly the protocols and metrics that make internet calling work.
| Acronym | Stands for | What it means |
|---|---|---|
| ACD | Average Call Duration | The mean length of calls in a given context — a simple but telling operational metric. |
| ASR | Answer-Seizure Ratio | The percentage of calls that are answered out of the total attempted. It’s a useful indicator of call quality and route health. |
| IAX | Inter-Asterisk eXchange | A VoIP protocol native to the Asterisk PBX, well suited to setups with limited bandwidth. It carries signaling and media in a single stream, uses Password Authentication, and works well through NAT and firewalls. |
| NAT | Network Address Translation | A technique that remaps IP addresses by rewriting address information in packet headers — rerouting traffic without changing the addresses of the hosts involved. It eased the shortage of IPv4 addresses and lets many private addresses sit behind a single public one. |
| PBX | Private Branch Exchange | The system that sits between a company’s internal phone network and the public network, connecting internal extensions (short numbers) to each other and to the outside world over the PSTN. |
| RTP / SRTP | (Secure) Real-time Transport Protocol | The protocol for delivering audio and video over an IP network. It takes over from SIP once a session has been set up, handling the actual media stream; SRTP is its encrypted form. |
| RTCP | RTP Control Protocol | RTP’s companion protocol. It monitors transmission statistics and quality of service, synchronizes multiple streams, and feeds back QoS information at regular intervals. |
| SDP | Session Description Protocol | A format for describing how a streaming-media session should start. It handles session announcement and invitation, and lets the two parties negotiate parameters such as which codecs to use. |
| SIP | Session Initiation Protocol | The signaling protocol that sets up communication over IP — voice, video or text. It makes sure each party can locate the other (by IP address) and agree on compatible codecs, then hands the media over to (S)RTP. In practice the term is used more loosely to cover IP voice in general, including SIP trunks and registrars. |
Where to go next
Telecom fundamentals — numbers, delivery receipts, character encoding, codecs — are worth understanding once and reusing forever. For the official, up-to-date detail on how these concepts map to the Callr REST API and webhooks, see the Callr documentation. And if you’re building voice flows, Callr Actions lets you assemble an IVR or a voice AI agent as a scenario without writing the telephony plumbing yourself.