[p_l]

One of the elements that came with mechanical (as opposed to human-powered) phone exchanges was introduction of "impulse billing" (there's probably a proper term for it) where establishing a connection triggered a device that printed a symbol every "pulse" from the timer on a ticker tape. This would be then collected, pulses counted, and you'd be billed based on the amount of those pulses. Similarly different devices would be able to generate "logs" of actions like "connected line A to line X", which would be collated and sorted regularly.

Different billing strategies often were based on how often the pulse was triggered. This is also related to "billing by the second" - it was originally a technological issue that caused per-minute billing.