The suction cup solution does not work very well. My wife recently had to record a podcast of a phone interview. I went to radio shack and bought a variety of devices that splits the signal, some at the phone jack and some at the handset with mixed results. Some worked once or twice and then stopped working.
Here is what worked the best. If you have a 2nd phone on the phone line you want to talk on that has a headset jack, plug a cable from the headset jack directly into the computer's audio-in jack. It will record both sides of the conversation on the other phone and the there is no interference from other devices.