Cisco 8851 Phones registering, but still not working

[JM_Robo]

So I have seen the multiple posts on here describing various issues with the 8851 Phones. I don't believe our issue is the same as most of these since myself and my team are working on these phones connected to a military network. 

A brief backstory as to why they stopped working:

DISA (Defense Information Systems Agency) decided to do a swap over of some of their equipment and didn't let us know. One of the things they changed was our phone block number e.g. we used to be 603 now we're a 604. Well we had to reprogram every phone on the base with the new number. We don't use a CUCM to manage the phones, we have our own boxes for this deployment for that. We just can't seem to get any of the 8851's to work at all. Can anyone give us a starting point to begin troubleshooting with the help of this community?

[Andrew Skelly]

Hi, JM.  Can you first describe exactly what is happening?  You say not working, but what exactly does that mean?  Unable to make/receive calls, no external calls, no internal calls, error messages when dialing, no dial tone, etc.  This will help the community in determining what steps you may need to take in order to resolve your issue.  Thanks!

[JM_Robo]

So we are deployed and are using models 7911, 7945, 7965, 7942, 8811 and
8851.

We do not run CUCM, we’re running SIP. We had a recent number block change
through DISA we updated the phones xml files to reflect the number change,
but some of the phones get stuck in registering.

We saw some recommendations to factory reset but this removes the phones
provision after rebooting.

Phones symptoms are no dial tone, but it registers an IP address and
immediately goes to an non-provisioned state. That’s for any phone that
gets the factory reset.

If there’s anything else you would need to gain more insight just let me
know. We’ve been at this for about a week.