There are still two different camps in VOIP technology. The most advanced is Cisco Systems AVVID, which combines voice video and data. If you would like to become technically savvy on this technology, the best place to start would be to learn 802.3 Ethernet as well as IP classless addressing. VOIP is only an application that runs on these types of networks.
Good Luck, and again there are many learning resources on these technologies on http://www.cisco.com. Most of what I have learned has been through there.
There are also some good books from McGraw-Hill on Telecom.
I have now deployed 11 IP Telephony PBXs with VOIP toll bypass over T1 and Frame Relay.