Key system

A key system is a type of telephone system designed to address the communication needs of small to medium sized businesses. They offer many more user features than a normal single line telephone set, such as placing a line on hold, multi-line support. intercom service, attended transfer, etc.

Unlike standard single-line telephones, which use a single wire pair to connect to a Central Office, key telephones require three wire pairs for each line. The talk pair is for the voice circuit, the A-lead supervises the talk pair, and the lamp pair powers the small incandescent lamp under the translucent key button at the front of the telephone. All three wire pairs connect to the the Key Service Unit (KSU) which interfaces between the key system and the outside telephone lines.