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SIP (Session Initiation Protocol) is a protocol that allows voice and video communication over the internet. SIP takes care of starting and ending phone calls, and does so in 2 stages: 1.SIP Registration
This establishes a connection and confirms that you are ready to make and receive phone calls. 2.SIP Invite
This takes place when you are actually making calls

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SIP Registration

To make a VoIP call you must first register your intention. Registration is made by your VoIP device, whether this be a softphone or a physical handset, and simply makes a connection to confirm that you are available. Registration isnt a one-off, with most phones re-registering every 1 to 60 minutes. This is known as the session length and set on the device itself. You can think of it of the device just letting the platform know that its still available and ready to make and receive calls. Sip Registration

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SIP Invitations

When you actually make or receive a call, it all starts with an Invite. The invite is between the end users device and the VoIP provider who is handling the call. During this invite they will establish:
  1. Codecs both parties are using and in which order to access them
  2. Which RTP (Real-time Transport Protocol) ports will be used for the transmission of audio in both directions.
Sip Invite

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What is a SIP Trunk?

You can think of a SIP Trunk as a virtual phone line. Instead of your phone call being carried over the copper phone network with a traditional line, its carried over the Internet on a virtual line. We refer to channels which limit the number of simultaneous calls you can make over these trunks. SIP Trunks require a phone system to convert your voice into digital packets to be sent across to the recipients network. It doesnt matter where the recipient is in the world, or what device or technology they use, the call will be made.