Back in HS, a friend and I built one to interview people for a project. We took an old phone and unscrewed the speaker end. We took off the wires going to the speaker and ran them into a transformer (or several, I don't remember) to get the signal hot enough to go line in to a tape deck. It took a few hours and a couple of trips to Radio Shack (remember when they were useful?) to make. It couldn't have cost more than $50.
We left that phone off the hook and used a different phone on the same line. Our voices were much louder than the interviewees - for whatever reason, the signal from inside the house is hotter than the one coming in from the phone line), so we just re-recorded our questions when we dubbed the interview to a master tape. Now, you could probably just fix that with software.