IP vs VoIP
VoIP introduces a collection of protocols and devices that allow for the encoding, transport and routing of audio calls over IP networks.
Voice ? IP ? Voice [P2P, Skype, Messanger]
Voice ? IP ? PSTN [Net2Phone, Deltathree]
Voice [PSTN] ? IP ? PSTN [iBasis, ITXC]
Voice [GSM] ? IP ? GSM/PSTN [???]