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When a phone line rings, there's 90 V RMS AC at 20 HZ on the line. It's enough to give you a jolt you won't soon forget. Thus, it's hard to miss!
My favorite detection scheme looks like this. This is off the top of my head so you may have to twiddle component values a bit. Also, this is for driving logic circuits. I'll treat your specific problem in a bit.
Detector Schematic
+-------------------------- + DC power supply | ^ CR2 | O---C1--+--R3---+--CR3>-+-------+-------+-----> ring det logic | | | | phone R2 ^ C2 R1 line | CR1 | | | | | | O---------------+-------+-------+-------+----- GND C1 .1 uf CR1,CR2,CR3 1N914 C2 10 uF R1 100K R2 10K R3 100K
When the ring signal comes along (90V RMS) enough voltage is developed at the juncture of R3 and R2 that some charge is pumped into C2 via CR3. Protective diodes CR1 and CR2 ensure that the output of this circuit will not grossly exceed the power supply levels and thus damage the logic circuits it may drive.
C2 and R1 have a time constant of 1 second, so one second after each ring ends, the output will fall to a logic zero again. This circuit could easily drive a counter, to count rings. A timer with a longer period could be used to reset the counter if no rings have come in within say 10 seconds.
For the specific problem in the post I'm answering, (" LED should stay ON while the phone rings") you would want to change the design somewhat. Here's how I would make a ring indicator light that stays on until the phone acutally stops ringing:
O---C1--+--R1---+--CR2>-+-------+--R2---+ | | | phone ^ C2 LED1 line CR1 | v | | | O-----------------------+-------+-------+ C1 1 uF, decent voltage C2 see text R1 10 K CR2 1N914 CR1 zener -- 9v or higher R2 1 K LED1 any old LED