I'm trying to pick an IP telephony technology to learn. I heard of Asterisk, trixbox, freePBX, and my head was already spinning being not sure what to learn. Then I came across this article listing some more like Kamailio, Yate, CallWeaver, FreeSWITCH, SipXecs and now my head REALLY is spinning http://www.cio.com.au/article/323016/five_open_source_ip_telephony_projects_watch .

Can someone give me a run down of how all these technologies tie together? What is the trend now, because I'd like to start learning.

Note: Anyone please re-tag this question if you know better, because I'm new to this field and not sure about the best tags.