I am currently working on setting up the voice notification with 3 different clients and each one has come back with the same question - one that I can’t find an answer to in the documentation or on the forums.
The question: is this a SIP trunk connection?
I’ve looked up some information on SIP and I have gotten a definition of what a trunk connection is, but that doesn’t tell me what type of connection Ignition is expecting. I suspect the answer is no based on a couple of posts I’ve seen involving “soft phones”, but I need a better answer than a hunch.
The technical answer is no. But then, neither is the majority of SIP hardware.
Ignition doesn’t care whether a SIP trunk exists or not. That’s all dependent on hardware-- what negotiates the VoIP session-- and the service provider. A ‘gateway’ of some sort still exists, even if they use a trunk.
Perhaps they need to provide what they are envisioning, if possible.
Here’s a white paper: SIP Trunking Deployment Models
It’s Cisco-centric, but the deployment models remain the same. 
I appreciate that. It looks like an interesting read. I am a complete novice at VOIP systems in general so any information to bring me up to speed is going to help out a lot.