Analog (POTS) telephone off-hook detection
You could also use a small moving coil meter (eg VU meter). They draw 50uA for full deflection, which IIRC is less that the 500uA you can draw when on-hook.

Put a flouro orange sticker on the needle as a flag, and cover half the meter face. When the meter falls, the flag is visible.







simulate this circuit &ndash; Schematic created using CircuitLab

Telepermit specification states (Can't see a US one, but I don't imagine it's much different)

6.7
On-hook line current
(1) The total current drawn by all equipment connected to a PSTN 
line within a customer's premises in the idle condition shall not 
exceed 1 mA.

(2) In the on-hook state the direct current drawn from a nominal 
50 V supply by any single item of equipment shall be not greater 
than 120 µA.

* The on-hook current drain should be as low as possible to allow 
customers the freedom to install several telephones without undue 
risk of exceeding the maximum value for the line.