I think Frog stated it best (I'm attempting a real answer):
1. Learn about telephony - how phone systems work, how it's organized, different types of switches, etc, etc. This will help you understand terminology. If telephony doesn't interest you, then this really isn't the right hobby for you.
2. Explore the system and try things out - don't just piggyback on exploits that other people have published. Exploration is more important than exploitation.
3. Keep in mind that many things (redboxing, blueboxing) no longer work in most of the country. It's important to know the technology used in your area and technology in other areas.