It would might cause feedback, but you could try a y splitter and a y combiner, something like this to split your headset, and something like this to combine the headset microphone with the phone out for the recorder in. They even have 3 way splitters you could use on the phone so you wouldn't have to listen through the recorder out, you would only have the headphones plugged into the recorder in, which might be less likely to create feedback. Just search under aux audio splitters or adapters and there are a ton of options, I just pulled the first 2 I saw. Good luck.