IP phones are really important for companies who make a lot of calls daily because with IP phones, the cost of the call will be cheaper.
IP phones usually belong to a telephone system. I can recommend Ozeki Phone System XE, Ive heard about it before and sounded good. http://www.ozekiphone.com/what-is-ip-phone-345.html --> this page can help you discover IP phones To understand IP phones better, you can watch a video about it.