PSTN
Public Switched Telephone Network
RFC 3666 Session Initiation Protocol (SIP) Public Switched Telephone Network (PSTN) Call Flows
RFC 3298 Service in the Public Switched Telephone Network/Intelligent Network (PSTN/IN) Requesting InTernet Service (SPIRITS) Protocol Requirements
RFC 2303 Minimal PSTN address format in Internet Mail
公衆電話系?
SIPと繋いでみたりできるらしい