This Instructable will teach you how to tap your own phone lines and record every call, which even works with cordless phones.This isn't the cheapest thing in the world to make, but it is awesome.
Step 1: Supplies
You will need to get a couple thing to do this.
1) Radioshack Recorder Control
2) A Recorder With Rem And Microphone Jacks (plugs)
3) Phone Line Obviously
4) 1-to-3 Jack Adapter If you have More Than One Phone
5) Extra Phone Cord
6) 8-Conductor Inline Coupler