Differences between IP telephony, internet telephony and normal PBX

Currently, the phone - this is one of the main items that people almost never let go of hands all day. The phone is constantly needed: at work, at home, on a trip, etc. Often there is a need to call another city or another state, and it is rather expensive. This is especially noticeable for large campaigns that constantly need to communicate with foreign partners. Sometimes the cost of a telephone connection can be tens of thousands of rubles per month. The question arises how to reduce these costs? What is IP-telephony and how does it differ from the PBX? Answers to these and other questions are discussed in this article.