As a kid I wound about 15 feet of #26 insulated wire on a nail and run the ends into the mic jack on a small tape recorder. I then plugged in a set of headphones into the headphone jack on the tape recorder and set it to record. MOST recorders are set up to monitor what is being recorded through the headphones. Obviously the one I used was too. I then seperated the red/green pair at a convenient place between the phone in use and the network interface. One wrap of the red or green wire around the my nail-coil and I could listen to what was being said on the phone. BOTH ends of the conversation. No one could tell or hear anything unusual on the line. It was a great way to spy on my older sisters gabbing on the phone. LOL I have to honestly say I never learned anything exciting or something I wasn't supposed to know by doing this but as a kid it was great fun.
Will what you described work? Not likely. I doubt any headphones you would have are sensitive enough.