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In other words, the union went years without negotiating compensation for being on call. My nonunion companies over the years all did compensate.

OP could just as likely mean..."when 24-7 was introduced, they demanded compensation."

24-7 isn't required if your org only depends on system between working hours.

24x7 wasn’t a big thing in the era of “data processing”, and things like mainframes were staffed 24x7,