The HP Q3093-80004 Adapter is a very simple, non-proprietary part. It has no internal resistors, nor does it swap polarity, as some have suggested. The Q3093-80004 is often erroneously referred to as a "Two to four wire phone cord adaptor" (do a Google search on it). It is not a two-to-four adapter, rather a 4-to-2 wire adapter (or in other words, a 2-line to 1-line adapter). It simply strips out the the second line (and third, if present) outside pair(s) on an RJ11 phone wire. This can be easily done yourself with a few common parts. I've seen these adapters selling online for around $15-$25, and have even seen some places priced at $50-$100+ for this adapter.
Here's how to assemble one yourself, for about a buck:
Get 1 RJ11 coupler (~50¢), and one 2-wire (single line) phone cord (~50¢). Plug your multi-line cord into the RJ11 coupler, then your 1-line (2-wire only) cord into the other side. Plug the end of your single-line cord into the device that requires the additional lines stripped out - HP fax machine, or whatever - and you will have stripped out the other line(s). Now you have your very own HP Q3093-80004 Adapter.
Final notes: I don't believe that the Q3093-80004 was created by HP just to sell adapters (as a few individuals have accused). I believe it was an afterthought work-around to poor initial engineering. This could have been easily fixed in the hardware in the first place, to ignore any second (or third) lines on the other pins.
Have a fanTECHtic day!
-J
