Page "Business telephone system" Paragraph 23

Into the 21st century, the distinction between key systems and PBX has become increasingly confusing.

Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations.

Nortel T Series Key System TelephoneThe modern key system now supports SIP, ISDN, analog handsets ( in addition to its own proprietary handsets-usually digital ) as well as a raft of features more traditionally found on larger PBX systems.

Their support for both analog and digital signalling, and of some PBX functionality gives rise to the " Hybrid " designation.