SIP
SIP is a signaling protocol used in communication between a Media Gateway Controller and a Media Gateway. If you want to upgrade the existing worldwide PSTN system and turn it to more robust, then SIP would be the first candidate.
It might be the best way to learn about SIP to look at examples of its use. Think of a scenario: Einstein from Germany called Newton in UK using SyncSwitch. The softswitch server of SyncSwitch (MGC) will make the communication possible by connecting with a SIP supported Media Gateway (MG) based in UK.
SyncSwitch begins the communication by sending "INVITE" message to the Media Gateway (MG). The INVITE message contains a FROM tag like "From: Einstein <sip:s.switch@synchronous.net> tag: 76431" and TO tag like "To: Newton<sip:ukgateway@domian.com>". You can see that SIP uses messages that are very similar to urls and email. That is why SIP is often called a text based protocol.
MG will response with "180 Ringing" message in response to "INVITE" to indicate MG has received the INVITE and alerting is taking place. If the requested gateway cannot be found over the internet, SIP gives a "404 Not Found" massage. You may see 404 not found massage in web browser if the requested URL does not exist. Here, you can see a SIP massage looks very much similar like an HTTP massage.
When the called party Newton answer the call; a "200 OK" response has sent to MGC. The MGC then sends "ACK" message to acknowledge "200 OK", at this point media or voice between Newton & Einstein begins to flow using another protocol like RTP (The Real-time Transport Protocol).This sequence from INVITE to ACK messages is like Q.931 connect signals of PSTN.
When any party; either Newton or Einstein end the conversation, a BYE request is sent to terminate the media session. The confirmation response to the BYE is a 200 OK.