Novice,
Actually there might be one way to accomplish this. I hesitated, thinking that the sound quality coming out on your headphones would probably not be as good as some other wired/wireless solutions. In fact I don't think the sound will be in stereo. Although you did mention that you wanted to listen to audio lectures as well.
The OBi has an interesting feature for doing Public Announcements (PA). This feature actually streams sound, in effect making the OBi an audio media streamer. Look it up in the User guide. You'll need a special cable to connect tel jack to audio device. I think you can pick one up on Amazon as well.
RJ11 Telephone Line Pass-Thru to 3.5mm Mini-Jack Conversation Recording/Line-out Adapter
http://dx.com/p/rj11-telephone-line-pass-thru-to-3-5mm-mini-jack-conversation-recording-line-out-adapter-18869Cisco Headset Adapter | PC Headset Adapter for Cisco IP Phones
http://www.youtube.com/watch?v=1cKjcTH6v14