I’ve used this little gizmo from Radio Shack for 20 years now. It’s the cheapest way I know to record a phone conversation. I use it for doing phone interviews. Some folks use it to record teleconferences, or some to archive their voice mail. (The laws about recording on the phone vary by state in the US.) You can buy expensive gadgets that do the same, but no better than this one. Radio Shack still makes it, and it is still cheap at around $26. You can plug it into a tape recorder, or digital recorder, or even your computer — anything that will take a standard 1/8 inch input plug.