Just some more information.
I purchased the Home Phone Connect expecting to use it with my ATT 5840 cordless phone system with answering machine. I had the same experience as dankaz. I wouldn't call this working although VZW does. So back I went to the VZW store where a helpful (I thought) rep told me that the HPC would work with newer answering machines. So I spent $60 on a new Vtech cordless phone with answering machine. Nothing changed. Back to the VZW store where another rep called an internal support line where he was told that Panasonic systems would work if you adjusted the flash timer. So I bought a Panasonic cordless phone system with answering machine ($120). Nothing changed. I tried every setting of the flash timer to no avail. Then searching the internet turned up a device (Viking Electronics CPC-1 $75) which claimed to detect the busy signal and generate the CPC disconnect. Hooked it up and NOTHING CHANGED! I called the Viking Electronics support line and asked them how many busy signals were required before the CPC disconnect would be generated. The answer was 6! More money down the drain. So unless you have a perverse definition of "works with your existing answering machine" you probably do not want a Home Phone Connect.
Why do I want to use the answer machine? For call screening, which you can't do with voice mail.