These days that's all a bit old fashioned - it's called a hunt group. Basically you set up several numbers and told the switchboard to call them one after another until it finds one that's answered.

It seems reasonable, but in practice by the time the network was dialling the second number the caller had given up.

It had to be that way because telephony could only have once instance of the call at a time. With VoIP you can send the same call to any number of telephones all at once so you just pick up the one you're next to at the time.

You do this by setting up your virtual switchboard to send the calls to whatever numbers you have which are allocated to individual extensions - you can then set the rules you like.

Voipfone do this for £0.99 per extension.

UK VoIP Internet Phone Service - VoIP PBX Switchboard and Telephony services