The setup I used isn't professional in any way. Here's a breakdown of it.
I have a normal PC with a microphone that is connected to the external portion of my audio card. The microphone input goes in to one of the effects channels in FL Studio. The voice is then edited (real-time) by some VST plugins. My computer's audio output is then split in two. One of the outputs goes in to an old '94 apple computer's microphone input, used by some phone software, and the other goes to my computer speakers (So I can listen to the call after it's recorded. They are turned off during recording).
The phone line is connected the apple computer. The audio output of the apple computer is also split in two. One of the connections goes to a pair of headphones so I can listen to what the person on the phone is saying, and the other goes directly back in to my computer so it can be recorded.