[ora]

Audio hijack is suitable, you'll also need some way to get the sound signal out of the phone. I do this for phone interviews in two different ways.
One is a little box you plug in between the handset and base of your phone which has an audio cable output (should be available from electronics shops), and the other is a single in-ear headphone with a microphone on the outside. You put it in your ear and hold the phone over it, and it records the sound coming out of the phone's speaker- while more expensive, this one allows you to record from mobile phones (cell phones), and is made by olympus.