Hi, I'm looking for a piece of hardware that will let me patch my
phone to my computer. My goal: to be able to have the phone on the mic
jack and the headset jack, so I can cleanly record off the phone line,
and (equally important) play sounds over the phone.

I've found several interfaces that will let me patch the phone to the
computer (or any mini-DIN jack, really) so I can record... but I
haven't found anything to let me play back over the phone.

Obviously radio stations, police departments, etc, use these all the
time. But I can't seem to find that equipment (in a consumer-friendly
form) anyplace.

I'm envisioning a very simple box, for under $80, that just plugs into
my phone line and has two cords coming out (mic and phone)... maybe
the "fancy model" also has some volume/gain adjustment knobs on the
front :-) I don't need any software to record or play back, just the
interface to give me a CLEAN connection to the phone line.

Help! What am I looking for, and where is it? :-) BTW I checked, my
computer modem won't do this. (Though the answer may be to buy a new
modem that will do this?)

THANKS--
Eric

Think that this will do the job for you.
http://www.ringcentral.com/software/learn-morePW.asp

I have an older version (2000) it has never let me down.

Thanks-- but one of the requirements is a "TAPI-compliant voice
modem." Which I don't have. Perhaps I should just buy one of those.
How clean is the recording and playback quality?

I would also really just like a piece of hardware (or a software
driver, if I go that TAPI route) so I can use my own sound software.
Looking at the specs for this, it doesn't seem like there is going to
be a *simple* *obvious* way to play a sound file over the phone, or
record the line.

I'd rather have Audacity running, the open source powerful sound
editing software, and just have it see my phone line as another input
and output source.

Ring central will do some of the things you are asking, they were
pre-loaded in many NEC machines, and are still avaiable as a
stand-alone software piece, but I thin you would be more happy with a
little external version from RadioShack. RadioShack offeres a
telephone recording device that has microphone and speaker leads. They
are designed to be used with cassette recorders, but you can convert
the mini plug connections with RCA adapters also at RadioShack. Using
the external device would allow you to record, play or talk during or
after the call with as good of quality as you would get from your
handheld phone itself.

Check out model #43-228 - they also have other items related by typing
"telephone recorder" into the search box on the radioshack website.
Hope this helps!