Linphone is an open source internet phone or Voice Over IP phone (VoIP). With linphone you can communicate freely with people over the internet with voice. Linphone complies to the SIP protocol, an open standard for internet telephony. Linphone should be able to interoperate with most SIP-compatible phones and proxies. If you subscribe a VoIP to PSTN (=classic telephony) account to a telecom provider, you can reach everyone that has a "classic" phone line.