You are probably talking about
Session Initiation Protocol that is used with VOIP.
It basically makes the connection to the person you are calling.
Like find out the IP of the user, authenticate your username with the SIP service, open the connection, ring the phone and on the end close the connection again.
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![[Image: 1-0.png]](http://tsdme.nl/yass/image/1-0.png)
