A single or double pair of wires is then run from each phone to the tenant demarc, and the phone company handles it from there. On the minus side, Centrex is expensive, especially for larger companies. US West, for example, demands a minimum one-year commitment on Centrex service. Later key systems incorporated electronic relays, but in operation they act much like their predecessors. If you ever get a chance to go through a phone junkyard, don't turn it down.
Key systems have every incoming business trunk appear as a button on every phone in the building.
Today, true key systems are completely obsolete, although you may occasionally see them come up in the used market sold for a little above scrap value prices. Hybrid Key systems Hybrid Key systems were developed to meet the demands of customers who wanted the reliability of transistors and a programming system that was easier than moving wires around in the mechanical key systems. Hybrid key systems maintain their popularity today by serving up the additional features of a larger PBX, such as voice mail, at the lower cost of a key system. Some of the newer hybrid key systems even have limited internal voice mail capability and can even take T1 and other specialty lines from the telco. Depending on the manufacturer, Hybrid Key systems can sometimes be fitted with analog cards and use regular phone sets.