How could she tape private conversations?
They used to sell suction cup microphones that you stuck on the back of your handset (listening end of the handset I think) that would plug into the mic jack on your cassette recorder. It would provide usable quality. Looks like they are still available on Amazon under 7 bucks (used to be a Radio Shack buy) https://www.amazon.com/Telephone-Microphone-Suction-Cup-Pickup/dp/B0034I75IK/ref=asc_df_B0034I75IK/?tag=hyprod-20&linkCode=df0&hvadid=198096917686&hvpos=&hvnetw=g&hvrand=3133852972130016671&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9026835&hvtargid=pla-392266572601&psc=1
After speakerphones became readily available, just put it on speaker and turn on the cassette recorder. I don't know how folks do it now.
Voice recorder for a phone line. The Feds were helping her. Here’s the link for the documentary we watched. I think there are four parts, but am not certain.
https://m.imdb.com/title/tt26753472/