Based on SIP (Session Initiated Protocol) a SIPphone is a SIP service that provides a free SIP-based VoIP network. To use SIPphone you need either a SIPphone adapter or softphone software and a broadband Internet connection. To make free in-network (SIP to SIP) calls, both sides of the call must have a SIPphone adapter or softphone. To make inexpensive calls to non-SIPphones (PSTN), you will need SIP minutes.
