Gentlemen:

Same in the UK

and

> I don't know how it is in Canada, but in the States, the 20Hz
> component is
> the only ringing voltage sent across the line to the
> telephone set,

both these are wrong.  Go and get a scope and have a look.  The proper
designation of ringing induction from CO to Callee (a telephone term
differentiating Caller [the originator of a call] from the person receiving
the call) is "20/1000"  which is composed of a 20 cycle sine wave modulating
a 1000 Hz tone.

Remember as well the ringing induction can be from Tip to Ring (normal in
North America), from Tip to Ground, from Ring to Ground, - I do remember
that the standard in the UK is different than here.

I did not speak to the pseudo ringing tone provided to the Caller.

A quick reality check, think through the reactance of a DC blocking
capacitor at 20 Hz and 1000 Hz, when you rectify the AC through the
capacitor and power a small relay from the result you are not working with
much of the 20 Hz component.

Larry
VA3LK