Oh My, another mistake in my dotage.

Yes, several are correct the ringing induction has a component of 20 Hz, but
it is a 20 Hz cycle of 1000 Hz that you get.  There are a lot of reasons for
this, but the critical; notion is that the ringing voltage peak is from the
120V Central Office battery, 120V and is AC at the 1.414 peak.

If you want put a scope on the ringing induction and take a look.  when you
put a .1 mfd cap in series with the line and rectify the ringing induction,
you are working on the 1000 Hz component, not the 20 Hz component hi.  The
1000 Hz component is what you hear when your listening on the line, the 20
Hz component makes the old mecanical bells jump hi.  The 20 Hz component can
also come in a number of variants.

Sorry I messed this up, very few understand how the ringing induction system
works, and i must put myself in the class of no longer having it down
pat....

Larry
VA3LK