The problem is that compterwiz's solution requires a phone with a headphone jack. An alternative solution--if you have a phone you're not using hanging around--is to take that headphone cord that you've already ruined and splice/solder it in place of the phone's earpiece... that is to say: open the phone up and cut the two wires going to the speaker that sits in the top, and connect those wires to the wires that go to your headphones. There should be two wires for each ear (four wires total), so connect the two insulated wires to one wire in the phone, the two non-insulated to the other.
If you want to simply listen and not be heard, you can just clip and tie off the wires going to the mic part of the phone, or you can install a mute switch or volume control of some sort.
Just an alternative idea, because I'm feeling helpful.