Glossary of telephony terms

Anonymous call rejection (ACR)

Feature provided by CLASS service of PSTN that enables the subscriber to reject all calls from callers who have blocked transmission of their caller ID sending them to a recorded message saying the called party does not accept anonymous calls and to unblock and try again.

Automatic Number Identification (ANI)

The delivery of the calling party's billing number by a local exchange carrier to any interconnecting carrier for billing or routing purposes, and to the subsequent delivery of such number to end users. Traditionally used by both exchange carriers and interexchange carriers to identify telephone numbers for billing purposes, and interexchange carriers for non-billing purposes, and customers of interexchange carriers and local telephone companies for customer account verification and other purposes. PBXs with special trunking arrangements can appear to the network as a central office switch and have access to ANI.

Caller Identification (Caller ID)

Call trace

Service that enables the called party to dial a special code after receiving an annoying or obscene call, for example, to instruct the telephone company to record the calling party's number, for possible use by law enforcement officials.

Automatic Call Return

For automatic redial of the last calling number received. If a called party is unable to answer the telephone while it is ringing, a special code (*69) may be dialed later by the called party to automatically redial the last calling number.

Caller identification services

Any services like ANI, CID, and CNAM that provide caller identification details.

Calling Number Delivery (CND)

Calling line Identification (CLID)

Caller can block.

Calling Number Identification (CNI)

Calling party identification (CPID)

Calling party number (CPN)

Central office (CO)

Computer telephony integration (CTI)

Custom Local Area Signaling Services (CLASS)

Dialed number identification service (DNIS)

Do not call registry (DNCR)

Feature group D (FGD)

Global system for mobile communications (GSM)

Standards for digital cellular networks.

Home location register (HLR)

Interexchange carrier (IXC)

Integrated services digital network (ISDN)

Digital transmission over PSTN copper lines. Up to 30 lines can be multiplexed on a physical phone line for transmitting data or voice. A separate channel is used for out-of-band signalling using the user part of SS7.

Local exchange carriers (LECs)

International mobile subscriber identity (IMSI)

Unique identifier for SIM identifying the user on a cellular network.

Mobile telephone switching office (MTSO)

Mobile switching center (MSC)

Mobile virtual network operator (MVNO)

Public switched telephone network (PSTN)

The historic network of copper wires and switches to transmit analog voice signals. This is in contrast to cellular and internet digital networks.

Plain old telephone service (POTS)

Same as PSTN

Primary rate interface (PRI)

T1 / E1

Private branch exchange (PBX)

Session initiation protocol (SIP)

Signaling system seven (SS7)

A signaling technique used by carriers to route and manage telephone calls. It uses out-of-band signaling, where information is transmitted on a separate channel, not in-band where it was shared with the voice channel.

Truth in Caller ID Act of 2009

Makes it unlawful for Americans to transmit fake caller ID for the purpose of defrauding, causing harm, or wrongly obtaining anything of value.

Session initiation protocol (SIP)

Signaling System Seven (SS7)

A signaling technique used by carriers to route and manage telephone calls. It uses out-of-band signaling, where information is transmitted on a separate channel, not in-band where it was shared with the voice channel.

Subscriber identity module (SIM)

Truth in Caller ID Act of 2009

Makes it unlawful for Americans to transmit fake caller ID for the purpose of defrauding, causing harm, or wrongly obtaining anything of value.

Visitor location register (VLR)

Used to keep information about the location of a mobile phone.

Voice over Internet Protocol (VoIP)