If you want to record to a computer off a standard telephone line using an USB device that can plug into a standard modular handset jack the price is usually about $200 and up, but all the ones I've seen use Windows based software. You can probably find the USB device only for less money, but finding one compatible with OS X might be a challenge.
You can buy an analog device, that plugs into the modular handset jack for about $60 and up. You then run the analog line out from the device into your soundcard on a Mac or PC and record the conversation using any audio recording software such as Audacity, etc.
A couple of examples, below. I have no idea how well these devices actually work (caveat emptor). But in theory at least , either should be
far superior to recording using a microphone place in a room with a speaker phone.
$60 analog device
USB recording device w/software.
If you're in the U.S., your local
Radio Shack store should have many different analog devices for recording phone conversations.
Google "usb phone conversation recorder" and you should get many, many results...