Glossary / Core Telephony Concepts
📞 Core Telephony Concepts covers the foundational terms you need to understand before exploring Cloud PBX, VoIP, or SIP. These are the building blocks of every phone system, from traditional landlines to modern internet-based calling. This section contains 19 terms.
On this page: PSTN · Analogue Telephony · Digital Telephony · ISDN · BRI · PRI · E1/T1 · FXS · FXO · Trunk Line · Channel · POTS · Dial Tone · DTMF · Caller ID · DID/DDI · Extension · Toll-Free Number · Telephone Number Portability
PSTN (Public Switched Telephone Network)
The global network of traditional telephone lines, switching centres, and fibre links that connects phone calls worldwide. When you dial a number from a landline, your call travels through the PSTN. It has existed for over 100 years and is still the backbone that connects most phone calls, even VoIP calls that eventually reach a traditional phone number.
Related: VoIP · SIP Trunking · POTS
Analogue Telephony
The original method of transmitting voice as a continuous electrical signal over copper wires. Analogue phones are simple devices that need no software or network configuration. They plug directly into a wall jack and work. Many older office phones and fax machines use analogue connections. Analogue lines are being phased out in most European countries as networks move to digital and IP-based systems.
Related: POTS · FXS · FXO · Digital Telephony
Digital Telephony
Phone systems that convert voice into digital data (ones and zeros) before sending it over the network. Digital telephony offers clearer sound quality and more features than analogue systems. ISDN was the first widely adopted digital telephony standard for businesses. Today, VoIP is the dominant form of digital telephony.
Related: ISDN · VoIP · Analogue Telephony
ISDN (Integrated Services Digital Network)
A set of standards for sending voice and data over traditional copper telephone lines in digital form. ISDN was introduced in the 1980s and became the standard business phone connection in Europe for decades. It comes in two types: BRI (for small offices) and PRI (for larger organisations). ISDN is now being retired across Europe, with Luxembourg and Germany phasing it out in favour of VoIP and SIP trunking.
Related: BRI · PRI · SIP Trunking
BRI (Basic Rate Interface)
The smaller version of ISDN, designed for home offices and small businesses. A BRI connection provides 2 voice channels (so 2 simultaneous calls) plus 1 data channel for signalling. In Luxembourg, BRI lines were commonly used by small businesses before the move to VoIP.
Related: ISDN · PRI · Channel
PRI (Primary Rate Interface)
The larger version of ISDN, designed for medium and large businesses. In Europe, a PRI connection provides 30 voice channels plus 1 signalling channel over an E1 circuit. This allowed up to 30 simultaneous phone calls on a single physical connection. PRI was the standard for call centres and larger offices before SIP trunking replaced it.
Related: ISDN · BRI · E1/T1 · SIP Trunking
E1/T1
Physical digital circuits used to carry multiple phone calls over a single connection. E1 is the European standard (30 voice channels at 2.048 Mbps). T1 is the North American standard (23 voice channels at 1.544 Mbps). These circuits were the physical infrastructure behind PRI connections. In modern systems, E1/T1 lines are replaced by internet connections carrying SIP trunks.
Related: PRI · Channel · SIP Trunk
FXS (Foreign Exchange Subscriber)
The port or interface that delivers a phone line to a device. It provides dial tone, ring voltage, and power. The wall jack where you plug in your phone is an FXS port. In modern VoIP setups, an ATA (Analogue Telephone Adapter) provides FXS ports so you can connect traditional analogue phones to an IP network.
Related: FXO · ATA · Analogue Telephony
FXO (Foreign Exchange Office)
The port or interface that receives a phone line. It connects to the telephone company's line or an FXS port. Your analogue phone has an FXO interface: it plugs into the FXS wall jack. In VoIP gateways, FXO ports allow the system to connect to traditional analogue lines from the phone company, bridging old and new technology.
Related: FXS · VoIP Gateway · Analogue Telephony
Trunk Line
A communication line that connects your phone system (PBX) to the outside telephone network. A trunk can be physical (copper, fibre) or virtual (SIP trunk over the internet). Multiple calls share the same trunk. The number of trunks you have determines how many external calls your business can make or receive at the same time.
Channel
A single path that can carry one phone call at a time. A trunk line contains one or more channels. For example, a BRI has 2 channels (2 simultaneous calls). A PRI has 30 channels. A SIP trunk can have virtually unlimited channels, depending on your internet bandwidth and provider plan.
Related: BRI · PRI · Trunk Line · Concurrent Call
POTS (Plain Old Telephone Service)
The basic, traditional telephone service delivered over copper wires to homes and businesses. POTS provides one line for one call. It is simple, reliable, and has worked for over a century. However, it offers no advanced features like call forwarding or voicemail without extra equipment. POTS lines are being retired as countries move to all-IP networks.
Related: PSTN · Analogue Telephony · Dial Tone
Dial Tone
The continuous humming sound you hear when you pick up a phone, indicating the line is ready for you to dial a number. In traditional telephony, dial tone is generated by the telephone exchange. In VoIP systems, the phone itself or the PBX software generates the sound locally. Hearing a dial tone confirms your phone is connected and working.
Related: POTS · PSTN
DTMF (Dual-Tone Multi-Frequency)
The tones generated when you press buttons on a phone keypad. Each button produces a unique combination of two audio frequencies. DTMF is used to navigate automated menus ("Press 1 for sales, press 2 for support"), enter PIN codes, and interact with IVR systems. It works the same way on analogue, digital, and VoIP phones.
Related: IVR · Auto Attendant
Caller ID (Calling Line Identification)
A feature that displays the phone number (and sometimes the name) of the person calling you. Your phone or PBX receives the caller's information before the call is connected and shows it on screen. Caller ID helps you decide whether to answer, forward, or ignore a call. In SIP and VoIP systems, caller ID information is carried inside the SIP headers.
Related: STIR/SHAKEN · DID/DDI
DID / DDI (Direct Inward Dialling)
A service that gives individual employees their own direct phone number without needing a separate physical phone line for each person. A company may have 100 DID numbers but only 10 trunk channels. Calls to any of those 100 numbers arrive over the shared trunks, and the PBX routes each call to the correct extension. DID is called DDI in some European countries.
Related: Extension · Trunk Line · DID Trunk
Extension
A short internal number (e.g., 101, 202) assigned to a person, department, or device within your phone system. Extensions let people inside the company reach each other by dialling a 3 or 4 digit number instead of a full phone number. The PBX manages extensions internally, so they do not use external trunk channels.
Related: DID/DDI · PBX
Toll-Free Number
A phone number that is free for the caller to dial. The business that owns the number pays for incoming calls instead. Toll-free numbers (like 0800 numbers in Luxembourg and Europe) are often used for customer service, sales hotlines, and support lines. They can be routed over SIP trunks just like regular numbers.
Telephone Number Portability
The right to keep your phone number when you switch providers. In Luxembourg and the EU, regulators guarantee number portability. This means a business can move from a traditional phone line to a Cloud PBX or switch SIP trunk providers without losing its phone numbers. The process usually takes a few business days.
Related: Number Porting · Regulatory
Related Sections
🔗 VoIP Fundamentals — Voice over IP, softphones, WebRTC, and packet switching
🏢 PBX Systems — Cloud PBX, IP PBX, Hosted PBX, and open-source alternatives
📡 SIP Trunking — How your PBX connects to the outside phone network
💰 Billing and Numbering — Pricing models, number formats, and porting fees
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