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BLF (Busy Lamp Field) A telephone may have a group of lights (LEDs) which indicate the status of extensions on your PBX. You can then see, at a glance, whose extension is busy, unavailable or free. Normally, you can press an associated button to call that extension directly.
CLI (Calling Line Identity) CLI or 'Caller ID' is where the number of the calling party/line is available to the called party so that you can see who is calling before a call is answered.
IVR (Interactive Voice Menus) An IVR is a system for automatically answering an incoming call, playing a pre-recorded message to the caller and then offering a selection of menu items, such as "press 1 for sales, press 2 for accounts".
Call forwarding Call Forwarding is a facility for redirecting an incoming call from its normally routed extension/destination to another destination or extension. Call forwarding can be applied to all calls, until the facility is switched off or can be selective, for example when the normal extension is busy/engaged ('forward on busy') or only after the call has been unanswered for a predetermined time ('forward on no-answer').
Call Diversion See Call Forwarding.
Session Monitor A session monitor is a facility provided by a PBX whereby an operator or other user can get an instant real-time display or report of the PBX status, for example active extensions or calls.
Call Transfer Call Transfer is the facility to transfer an active call on one extension to another extension. Call Transfer can be attended, whereby the original extension user announces the call to the new recipient before putting them through or unattended, whereby the original recipient hangs up immediately after transferring the call and the call rings the new extension directly.
CDR - Call Detail Records CDR is a list or log of calls made or received by extensions on your PBX, or by your PBX trunk itself. It will normally, at the very least list the time/day of call, destination and duration. It may also include extension numbers, Caller IDs and call cost.
Intercom Normally, any extension on a PBX can call any other by dialling its extension number (or pressing a hot key corresponding to that extension). The two parties can then speak to each other, which is known as an intercom facility. On an IP PBX, Intercom calls are carried completely over IP so the calls are free, even if the extension is on the other side of the world.
DND (Do Not Disturb) This is a facility whereby an extension is set to reject incoming calls and the phone will not ring until the setting is turned off again so that the user is not disturbed for the duration.
DID/DDI A DDI number normally refers to one of several phone numbers where each one is directed to a specific extension or department, in addition to their main incoming phone number. If a company therefore has several/many DDIs, a caller can then call directly to a department or extension without having to go through the main reception or other departments.
Call Barring Call barring is the facility to block calls based on rules which you select. You can, for example, bar all calls, so that a group or extension cannot dial any calls or block dialling of specific destination (e.g. mobile, international or premium rate numbers). Different rules can be set for different extensions or groups. Incoming call barring can also selectively block all calls or calls from specific locations.
Remote Extension A remote extension is one which is not in the same physical location as your IP PBX, main office or other extensions. It will be connected using IP over the Internet or through a VPN tunnel to your IP PBX. A remote extension on an IP PBX works just like a local extension, including intercom, hunt groups and BLF facilities.
Call pickup (Extension or Group) Call Pickup is a facility which lets you answer a call which is ringing on another nearby extension, even if it is not ringing your own extension. This is ideal for where a colleage at an adjacent desk has gone away from their desk for a moment.
Voicemail Voicemail is a facility for the PBX to play a pre-recorded message to callers and then let them leave a message. That message is then available to the person who was unable to take the call, either by listening from their extension handset when convenient or the message can be forwarded by email for listening elsewhere. Voicemail can take a message when an extension is busy or when unanswered after a period. Voicemail can be applied to individual extensions or groups.
MWI (Message Waiting Indicator) A Message Waiting Indicator is a light or indicator of some kind, typically on your phone which lights up to indicate the presence of a message left for you on voicemail. Once that message has been listened to or deleted, the indicator will turn off.
Music on Hold Music or pre-recorded messages which play to callers when they are put onto hold.
SIP (Session-Initiation-Protocol)

This is used for the signalling and control element of SIP Voice over IP. This typically operates on UDP port 5060 and is used to pass messages such as call INVITE packets to phones to arrange the details such as the Codec used and the IP address to send audio data to. The voice data element of a SIP VoIP call is RTP.

SIP messages are also used to pass status messages from the PBX system to extensions so that the Busy Lamp Field information can be updated.


This is an abbreviation of "Coder-Decoder", which is a method to encode and decode data. In the context of a PBX system, they are used to encode analogue audio into digital signals to transfer over an internal network or the internet and decode the digital signals back into analogue audio to play back on a Voice over IP handset. Each Voice over IP device will support a number of codecs and each codec has different charactistics.

  • G.729 is a low bandwidth codec which compresses voice data to 8 kilobits per second
  • G.711 A-Law is a high-quality voice codec which is used by UK and European ISDN systems. It operates at a data rate of 64 kilobits per second
  • G.711 μ-law is similar to A-Law but is used in North America
Hunt Group A Hunt Group is a group of Extensions that allows a single call to be put through to multiple Extensions by dialling a single number. Calls to a Hunt Group can be put through to all extensions at once, randomly, all at the same time, or can go through a queueing system.
Auto Attendant See IVR
Answerphone See Voicemail
Extension An Extension is a Voice over IP client that is registered to an IP-PBX system. When the Extension Number is called, the PBX system puts the call through to the handset registered to that Extension. Each Extension has its own configuration such as permissions to call out and actions taken when the handset is busy or is unable to take the call in time.
FXO Port A physical port on a PBX into which you can connect a standard analogue line in order that the line is accessible by the PBX. It might also be referred to as a 'line port' or 'analogue trunk connection'.
FXS Port An physical port into which you can connect a standard analogue phone. It might also be referred to as an 'analogue extension port'.
Dial Plan

A Dial Plan is used to determine how calls made by Extensions are routed by the PBX system. If the call can be routed locally, for instance to another local Extension, the Dial Plan would not take effect.

If the number dialled is not available locally, the PBX system would check through the Dial Plan configuration, with elements such as Digit Map, Speed Dial and Call Barring, to decide where to route the call to and whether the call should be allowed or not.

Digit Map The Digit Map facility is used to route numbers dialled for out-bound calls, based on the criteria specified in the Digit Map entry settings, such as length or matching prefix. It is also able to modify the number such as altering or adding a prefix based on the prefix dialled or the length of the number dialled.


PSTN (Public Switched Telephone Network) This is the international voice / telephone network.
POTS (Plain Old Telephone Service) Another term for a telephone line that provides voice connectivity.
Analogue Line An analogue telephone line passes sound through the telephone network using analogue audio signals (voltage levels) instead of encoding the audio. This limits the use of the line to a single call per physical line installed, additional calls would interfere with an existing call.
RJ45 (Registered Jack 45) Standard network connector, typically used for ISDN systems and Ethernet networks.
PoE (Power over Ethernet) This passes power over an Ethernet connection along with data, typically at 48 Volts DC with up to 15.4 watts of power. This will typically be implemented using the inter-operable standard IEEE 803.2af. This can be used to power IP phones or other networked devices while passing data over the same cable, to simplify cabling or install IP phones in locations that do not have power available.
ISDN (Integrated Services Digital Network) A system for passing both data and voice calls digitally over the existing PSTN network. This allows two or more calls to pass over the same copper cable that an analogue line uses by digitising audio and sharing the line between two "channels" with Time Division Multiplexing, which is then reassembled into PSTN audio or data by the telephone company. The digital nature of ISDN also makes it possible to provide additional features, such as MSN Numbers (multiple phone numbers) through the same ISDN line.
B-channel (ISDN) This is a Bearer Channel, which operates digitally at 64kbps in both directions. This carries either data or digitised voice data which is then routed by the telephone company to the PSTN network.
ADSL (Asymettric Digital Subscriber Line)

Broadband over standard copper wiring which communicates with the exchange using an ADSL modem. Initially provided up to 2mbps downstream and 256kbps upstream which increased to 8mbps downstream and 512kbps upstream. This was later increased to up to 24mbps downstream and 1mbps upstream with ADSL2+. The speeds possible vary depending on distance from the exchange and line quality.

Different countries use different Annex types for ADSL connections; the UK uses Annex A, which is ADSL over an analogue connection. Annex B is used in countries such as Germany and provides ADSL over a digital (ISDN) line.
The different Annex types are not inter-operable so it is not possible to use an Annex B router with an Annex A ADSL connection.

SDSL (Synchronous Digital Subscriber Line) Similar to ADSL but with an equal portion of frequency assigned to upstream bandwidth, which results in 2mbps upstream and downstream.
ADSL Annex M Annex M is a modulation subset of ADSL and ADSL2+ (over Annex A or Annex B) that assigns more frequency to upstream bandwidth, increasing the potential upstream bandwidth to 2mbps while slightly reducing downstream bandwidth from a normal ADSL/ADSL2+ connection.
Traffic Shaping Also known as Quality of Service, this is a method for prioritising one type of traffic over another. This is often performed by ISPs to prioritise higher priority voice data over bulk data such as streaming video downloads.

The Contention of a service signifies how much bandwidth is available to the ISP compared to the number of subscribers that could potentially be using that bandwidth. If the contention ratio is said to be 50:1, that means that the amount of bandwidth available to the ISP has 50 times the possible bandwidth usage by its subscribers. This means that although the internet connection may connect at 50mbps, the ISP throughput could go as low as 1mbps if all customers were to make full use of their internet connections at the same time.

Jitter A term used to describe a variation in the delay from receiving one packet to the next. If a connection has no 'Jitter', all packets will arrive with the same delay. If a connection has significant 'Jitter', the delay from receiving one packet to the next can vary significantly and may be unpredictable. In the application of Voice over IP, this would potentially cause a break in a call's audio stream. This is often improved with a 'Jitter buffer', which adds a small delay to allow for some 'Jitter' to occur. This would then be sent in order with any packet delay corrected.
Packet Loss Is used to describe data packets that are lost in transmission across the internet. If packet loss affects the audio data stream used by Voice over IP, this would cause a brief loss of audio or other noise distortion to a call.
Throttling See Traffic Shaping. Throttling is traffic shaping applied to reduce bandwidth available to specific IPs or services, to reduce the throughput of that traffic.