also, it was constrained by a queue. Usually 1 phone line therefore only one person could be on at a time (cooperative multitasking if you will). busy signals

also, it was constrained by a queue. Usually 1 phone line therefore only one person could be on at a time (cooperative multitasking if you will). busy signals (when was the last time you got a busy signal anywhere?) and auto-redials. good times!