SIP calls and the ITSP not explained

Hey guys,

I have been using Freepbx and Sipstation for some time, and up until now, I have just assumed how the SIP technology worked. I was watching a video that explained how SIP Trunking works, and it said that inbound and outbound calls always go through the ITSP/SIP Trunk.

I had always thought the SIP trunk, or ITSP, just acted as a DNS-like provider, that would connect our PBX system to other SIP-based PBX systems(directly), except when the caller is using a landline. At which point the call would have to go through the ITSP (SIPSTATION or affiliates).

I assumed this because when I make and receive calls, and view “SIP DEBUG ON” from the console, it always shows random IP’s that it’s connecting to. But, if the calls always go through our ITSP, then shouldn’t it just show the IP of my ITSP?

In fact, on almost every call I make and receive, it shows a caller with an IP, even when calling cell phones etc. The only thing I can think of is, maybe the ITSP has many locations/IP’s, and it connects to the closest location of the other party.

Can someone explain? And if some calls don’t go through the ITSP, do you still get charged minutes for this?

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