Phreaking is my favorite topic right now. I've been working with phones for a decade or so and recently set up a phone server of my own to mess around with. Instead of just letting it sit there unused, subscribers to my podcast get a password to the party line. I'm even going to be releasing a choose your own adventure game on the phone system for subscribers in february or so. There's even a rough version of an phone audio museum if you call in. All of which I'm running on a raspberry pi at the moment, and admittedly need to polish off. It's a super fun hobby, though.
You definitely can't do a lot of the things you used to be able to, but I feel like there are features in freepbx and asterisk that make phones extremely fun.
edit: also, this site is incredibly interesting for those interested in more info http://www.wideweb.com/phonetrips/