Sure...to get a head start, see if you can find a Radio Shack Headset/Handset Switch. They stopped making them last year. Thankfully, I managed to buy two. It was a great device...you put in a couple batteries, plug it in between your desktop phone handset and the phone body, then plug in any 2.5mm phone headset. And it only cost five dollars. Pity they stopped making it, that was the only useful product in the store. Of course you can buy a system with the same function and Plantronics branding for upwards of 99 dollars.
Anyway, that would be a good way to get access to the telephone audio. Next, the device could route the signals to 3.5mm jacks so you can use a regular PC headset and boom mic for the telephone. Pipe in stereo music, maybe use an analog mux to switch and mute sources. I want to build one that senses the phone ringing by watching the activity LED, and automatically mutes music until the call is over. And lights a big "ON PHONE" sign over my desk.