So here's the deal:

I have an old phone headset (with a mic and headphones) that is a 3.5mm jack, and I want to use it to record to my mac. i.e. teamspeak. However, I only have a line in and line out on the mac, and if if plug the headset into the line out, the headphones work, but not the mic, and if I plug it into the line in, neither the headphones nor the mic work. Is it it all possible to make this work?

You need an adapter to convert TRRS to two TRS jacks. Search for the product yourself to get a decent deal.