What technologies are used or needed when implementing or using VoIP?
Basically you need an IP-PBX, routers and IP phones either hardware or softphones which are software based. You have some choices in technology, but SIP appears to be the most promising. The white paper reference above includes explanations of how VoIP works. If you have follow up questions, please let me know.
You should also review this previously answered member question: https://searchnetworking.techtarget.com/answer/Im-interested-in-learning-about-VoIP-Where-should-I-start