Reachable/unreachable is not a registration issue. The PBX is not receiving 200OK packets back from the device after sending an OPTIONS packet. Usually this a misconfigured router/firewall in between.
SonicWALL calls it “Consistent NAT” under the VoIP section - don’t know what kind of router the PBX is behind, but something like this is what is causing the problem.
@lgaetz Thanks for your reply. FreePBX is behind a pfSense firewall, and I believe the remote phone is behind a pfSense firewall also. There are a number of remote phones connected to FreePBX and they aren’t showing this issue, so I’m going to assume that the issue is with configuration on the remote pfSense firewall, and I will work with a colleague to resolve the issue.