edtomfish,
Alright, let's start at the beginning and see where things are at.
Under System > Devices and Feature Codes > IP Connections: Set the Base IP connections Public IP Address
Under System > Devices and Feature Codes > IP Connections: Set the PEC IP connections Public IP Address <-- This requires a second public IP Address or audio won't work if you don't have a PEC ignore this step.
Since you say that you have a full 1:1 NAT then opening ports is not the issue, but some firewalls still do block information if not setup properly.
To get a phone to come up it only requires connecting to the Base IP Address. The PEC is only going to effect audio not the phone functionality.
So, we know when a phone tries to connect it will boot up and look at the Public IP pointed to the Base NIC. It will look for TFTP [UDP: 69 or 20001] to see if it needs a new software load. It will then look to make a MiNet [TCP: 6800-6802] data connection to the Base NIC. At this point the phone should come up and start working as MiNet is for Call Control, but without SAC [TCP:3998-3999] HTML and DSS/BLF functions won't work so you should see those coming from the MiVO-250.
It sounds like your ports 6800-6802 are not getting through in both directions so I would look at that first.
Once it is up then you will need to worry about your audio getting through.
Base NIC: UDP: 6004-6261
PEC NIC: UDP: 6604-7039
You will need two different public IP Addresses if you have a PEC there is no way around this!!!
I know that Tech1302 has a few more ports on their listing, but some of those are for different style phones.
5004-5007 = Inter-Tel IP Endpoints
50098 - 50508 = Mitel IP Endpoints going out which usually does not have to be opened up unless the firewall is really locked down.
Also as you know you will need the phone to be setup as NAT; which you already stated.
Hope that helps.
Thanks,
TE