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These microphones are available at electronic and phone stores. Their great advantage over other physical methods is their small size and low profile. Install it into your receiving ear so that it will pick up the sounds from the speaker when you answer your phone. Plug the audio jack of the microphone into a portable recording device. Pocket-sized digital and optical media recorders are widely available for purchase at electronics stores and online. Switch your portable device's recording on as soon as you get a call. The microphone is always on and will send the sounds it picks up to your recording device.
Use an in-ear microphone. Wear the microphone. Plug in the microphone. Record your call.