VoIP and SIP

I am a newbie trying to wrap my head around VoIP and how it works. I work at an office as an intern and they use VoIP with their headsets and phones. I was just wondering if anyone knows of any information or websites, books that help explain it a bit more in depth. Also, I was wondering what a SIP Pager adapter is and how to install it and also how it all works. It is a lot of information and I am new to the whole VoIP thing so if anyone can help to explain this, it would be much appreciated. Thank you.
 
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