Where can I find a phone line off-hook indicator for sale? Something with an LED light to indicate line in use

Question

Something very simple, perhaps just a LED light on a small plastic box powered by a battery that has a phone line in and out (so as to be placed in-line) that lights use when the phone line is in use so nobody picks up the line. It will be used on a fax machine line with multiple fax machines so that someone doesn't initiate a fax while someone else is using the line.

Answer 1

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