I actually lost my headset adapter with a mic so I am in the process of ordering a new one. Below are a couple of examples of the adapter you will need to accomplish what you are looking for (listen to music, then answer call while headphones are still in use). Let me stress that I have nothing to do with either seller and can't verify quality etc, although I have had good luck with these types of items through ebay.
http://cgi.ebay.com/ws/eBayISAPI.dll...m=270430076463
http://cgi.ebay.com/ws/eBayISAPI.dll...m=350183272407
In my experience, a US buyer purchasing from Hong Kong will take about 3 weeks to arrive, so if you are in a hurry a US seller would be quicker (although a little more $). In either case, you can get the adapter pretty cheap.