Here's a site with just about all the DR devices I know of:
http://www.telephonesystems.com/accessories/line_sharing_devices.cfm
I have one by COmmand COmmunications. I would not recommend it. It
answers the line and chews up one ring... then after it rings the DR
line and the answering machine picks up, it throws one final ring on
the other (primary) line. Weird. Never worked right. I talked to
COmmand before I bought it and they insisted it would see the ring
pattern without actually chewing up a ring cycle but it does not work
as claimed.
I used to have two very simple filters you just plugged in like DSL
filters with a setting for which ring pattern (1,2,3) they let
through. No power required, worked off the line. Worked great. Very
inexpensive. Unfortunately I lost them in a direct lightening strike
and the company making them stopped doing so... that's when I got the
CC (lame) device.
If you build those filters drop me a line, I'll buy one