How to record a landline phone-to-phone conversation using a MacBook's audio-in port?

Question

I need to record a phone conversation -- that will take place between two landlines.
(I have the other person's permission -- I'm interviewing them for a paper and we both want a permanent audio recording of the conversation so that the quotes can be later double-checked for accuracy. So, the legality question is not the issue.)
I could do it the old-old-old-fashioned analogue way of getting one of those 1960's-style microphone pickups that have a suction cup to put on the phone...but the audio quality of that is probably pretty low.
I'd much prefer doing it directly digitally by having the phone's audio signal go into my MacBook Pro's "audio in" port and then recording the whole thing with an application on my Mac.
I've solved various peripheral aspects of the problem already: I have a choice of at least three different applications that can make recordings of the internal audio signal on the Mac (among those applications are Amadeus and QuickTime Player 7). And I have a "portable handset" connected to the landline, and the handset has a small audio-out/in jack into which a headphone headset can be plugged. And I have a converter plug which converts a "larger" (I thinks it's 3.5mm) computer-style audio jack into a "smaller" (I think it's 2.5mm) phone-style audio jack.
My questions are: Can I just use any available random 3.5mm-to-3.5mm male-to-male audio cable (such as those that connect tabletop computer-speakers), place the converter plug on one end and insert it into the phone, and then plug the other end into the MacBook Pro's "audio-in" port?
What would happen if I tried that?
Or do I need some kind of specialty cables, or specialty plug-converters?
My MacBook Pro is from about 5 years ago and has one of those "audio-in" (also sometimes referred to as "audio line-in") ports that depict above them a symbol that looks like a circle with two triangles overlapping it (clear image of what the port's symbol looks like here: http://www.kozco.com/tech/audacity/pix/MacLineIn.jpg).
Or do I have the wrong hardware/port to achieve this? If so: What steps can I take to make a digital recording of a landline phone call successfully?
Presumably, even if this scenario I sketch out above is feasible, since the audio out-signal from the phone is being redirected to the phone's out-port, I will not be able to hear the person on the other end from the phone's ear-piece itself; instead, I'd have to speak into the phone's mouthpiece but to hear the other person I must turn on the computer's volume and listen to the audio coming out of the Mac's built-in speakers. A little awkward, but doable.
Or am I barking completely up the wrong tree? If so -- what is the right tree? I'd like to do this without buying more hardware or specialty cables, if possible.

Answer 1

Quick update, for those interested:
Surprisingly, I have gotten it to work using just a random cable and converter plug, despite the predictions that it simply could not work. Well, to be be precise, it sort of works -- I'll explain what happens:
Taking a random 3.5mm-3.5mm male-to-male audio cable from some old tabletop computer speakers, I plug one end into my MacBook Pro's "audio in" port; the other end of the cable I plug into my "3.5mm-to-2.5mm converter plug" that I mentioned in the original post. Then I stick that converted end into the 2.5mm audio-out jack on the side of my Panasonic portable handset (for a landline). Then I open System Preferences > Sound > Input and make sure "Audio Line-in port" is selected. Then I launch QuickTime Player 7 and open Preferences>Recording and make sure "Built-in Input: Line In" is selected. Then in QuickTime Player 7 I go to the menu bar and choose File> New Audio Recording. Then if I make or receive a call on the phone, and press the "Record" button in QuickTime Player 7 -- it records the conversation perfectly clearly!
Well, sometimes it does.
The problem is that the converter plug tip that fits into the phone's audio jack needs to be fiddled with for the audio signal to be transmitted correctly. In fact, strangely, it DOES NOT WORK if the plug is plugged in all the way! For reasons I don't understand, it has to be sort of loosely and delicately half-plugged in for the connection to work correctly. What makes it difficult is that it's nearly impossible to keep the plug plugged in at this delicately balanced halfway level. The slightest motion will make it either go in further, or come out further, and either way, the audio signal is cut off, and I have to fiddle with it to get the connection back by making the plug get back to its perfectly balanced mid-way position.
However, when it is getting the signal, the sound is normal volume -- not dim or unamplified. That's the good news.
I think the problem lies with the exact nature of my converter plug.I notice that most other audio plugs, including both 2.5mm and 3.5mm, have three "bands" of metal on the shaft, separated by two lines (usually plainted blue or black). From my meager understanding of what this means, one of those bands is for incoming audio, the other is a "microphone" band for outgoing audio, and the last is the "ground." Yet the tip of the converter plug I'm using has only TWO "bands" of metal separated by a single black line. So I think it is lacking the "microphone" aspect, and the reason I have to have it plugged only partway in is so that the two bands are trying to match the connectors for the three bands deep in the jack hole.
I will try using it this way today, but for future attempts, next time I'm near an electronics store, I will try to find and buy a 3.5mm-to-2.5mm converter plug that has THREE bands, including the "microphone signal" band. Once I have that, I predict it will work perfectly!
If I ever get to that point, I will update this thread.