What I used to do for my own PA system is use the sound card in the old PC I had running Asterisk.

Use chan_oss or chan_alsa, and the Asterisk box takes care of the automatic picking up. It works great.
Grab a spare PC, install asterisk, do a IAX channel to it and it think you'll find it better, have more control.

The line in my asterisk setup was :
exten => 310,1,Dial(Console/dsp,10,A(tone11)) ;;intercom

This dialed the Console, or sound card, then played a tone right before the channel went live, as a warning tone.
There's tons of applications for this. I used to have a program that somebody made that would say the incoming callers
Caller ID name and number over the intercom as the phones rang. Plus, you could dial in mp3 requests and i'd use mpg123.

Good stuff. I'd recommend going this route. you don't need a huge, fast new PC, I did it with a P2 450, and it sounded great.

My dorm doesn't have air conditioning, but we do have a large ice machine in the basement.. if I could just figure out how to get rid of the water without ripping a hole in the window screens, this might actually work. Of course, I also have to deal with where to put a garbage can full of water in my tiny room and convincing my roommate that the 5 degree difference is worth the eyesore...

On second thought, maybe I'll just go visit those apartment friends I've been neglecting.. the ones that have AC and unlimited utilities...