Telephone terminology

Telephone

The device used to make and receive telephone calls. One telephone generally allows one person to make one telephone call at a time. While this may seem obvious, there are some things to be aware of that are not immediately obvious.

  • There are cordless telephones with more than one cordless handset. Although they may have two or more handsets, they are still only one telephone.
  • There are telephones, both corded and cordless, that can be connected to 2 or more telephone lines. These telephones allow more than one telephone call to be made at a time.
  • Computer software known as a 'soft phone' can be used to make and receive telephone calls using a computer.
Telephone handset
The part of the telephone you hold against your ear and speak into. A handset is not the entire telephone; it is only the part that you hold in your hand. A mobile telephone is both a telephone and a handset.
Telephone headset
Like a handset but worn on your head instead of held in your hand. Useful for people who are on the telephone for long periods of time.
DECT (Digitally-Enhanced Cordless Telephone)
A cordless telephone that uses a digital radio link between the base and the handset. Most if not all cordless telephones currently made are DECTs.
Extension
A telephone connected to a telephone system. If you have a telephone system, telephones are not connected directly to telephone lines. Instead, all telephones and lines are connected to the telephone system. This allows any telephone to use any line, and provides features such as hold and transfer. It also means you only need as many lines as calls you expect to be in progress at any one time, which could be far fewer than the number of telephones you require.
Telephone line
The external connection from your building to the telephone network. One telephone line allows one person to make or receive one external telephone call at a time. Multiple telephone lines can share the same telephone number.
Telephone number
The number that others use to contact you. One telephone line can have one or many telephone numbers.
PSTN (Public Switched Telephone Network)
The telephone network available for the public to use. There is a common misconception among some service providers and even some BT engineers that 'a PSTN' is an analogue line but this in fact incorrect. PSTN refers to the telephone network rather than an individual line, and includes all circuit-switched lines, such as analogue lines, ISDN lines, and even the mobile telephone network. The only thing not included is SIP trunks, which are packet switched.
Analogue line
An ordinary telephone line suitable for one ordinary telephone, such as you would have at home.
ISDN (Integrated Services Digital Network) line
A digital telephone line that can carry data as well as speech. Digital lines are far superior to analogue lines as there is no loss of speech quality.
ISDN 2
A type of connection that provides 2 digital telephone lines in one connection. ISDN 2 is generally used for between 2 and 8 telephone lines. It was originally used to provide a connection to the internet, but this has since been replaced with ADSL.
ISDN 30
A type of connection that provides between 8 and 30 digital telephone lines in one connection, or multiples of 30 telephone lines in 2 or more connections.
VoIP (Voice over Internet Protocol)
VoIP is a technology whereby telephone calls are routed over the internet instead of over traditional telephone lines.
SIP trunk
A type of telephone line that uses VoIP.
Call forwarding (or call diversion)
Rerouting an unanswered call to a different telephone number.
Call transfer
Rerouting an answered call to a different telephone number or extension.