This is the setup I use. The black phone has an inductive pick-up coil like the kind they sell at Radio Shack. This one's glued onto the ear piece. It operates completely through the field induced by the speaker, so no assembly required.
The cord plugs straight into the tape recorder with one of those 3.5mm mic jacks. I've had it for months and the audio quality is pretty great on both sides of the conversation. Depending on your local laws, you might find them at any electronics retailer.
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