Dunners,
Yes, the E.164 is where you are getting the plus and in this scenario that is not needed.
It seems as though your MiVO-250 has a PEC is that correct? I am seeing both of these addresses within your SIP messages; would this be correct?
MiVO-250 Base: 192.168.96.1
MiVO-250 PEC: 192.168.96.2
Also, your SIP Error Code 500 Internal Server Error is usually due to the side sending the code not having a trunk available. Have you programmed a trunk on the Free PBX yet? I see that you only have 1 trunk programmed on the MiVO-250 currently so you can only make one call. Don't mistake Registration and Trunks in Service as a sign that there are trunks available on both sides; it just means that the systems were able to register to one another. The 500 error could also mean that there wasn't a Route to the other location, but it seems the message came from the Free PBX so I would guess the former rather than the latter.
Thanks,
TE