UniXcape is an internet phone (VoIP), compatible with the SIP protocol.
If you subscribe a VoIP to PSTN (=classic telephony) account to a telecom provider, you can reach everyone that has a "classic" phone line.
Alternatively.
UniXcape available on UniXcape Member only.
List of supported features:
-Audio with the following codecs: G729
-Vdeo with the following codecs: H264
-Chat message
-multiple calls
-audio conferencing
-Address book integration
-Call history
-DTMF (telephone tones) support using SIP INFO or RFC2833