VoIP call functions like the regular calls, that is the voice is sent to another person and vice versa. but the VoIP phone system transmits your voice over the internet in form of a digital data packet. with regards to the network equipment, there is actually no difference between voice packets and those which have other kinds of data. rather than dedicated copper wiring between the callers VoIP technology uses existing data networks. in simple words, VoIP calls are cheaper for the same reason that an email is. they do not require any separate network infrastructure just for voice. this system depends on networks which are already used for other tasks. since you already pay for an ISP for internet access therefore now you know that there are free and cheap VoIP calls which are not just a marketing trick.