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I finally broke down and opened a support case and the response was incredibly simple (and verified as working). See below:
Our recommendation is to disable registration on the PBX as it doesn't provide any benefit. If registration is a requirement, you can do the following
voice user 0000
sip-authentication password <password>
It is critical that the voice user is not a valid phone number as that will cause call routing issues. The PBX would then register the user 0000.
Note that the WAN trunk registration is completely independent from the PBX's registration.
I tested both ways:
PBX not registered, calls work, no issues
PBX registered, works, no issues.
Hope this helps, if you need it.