Fix Grandstream Inbound Calls

I have a Grandstream UCM6300A that is happily sending outbound calls, yet any inbound call never reaches the phones—there’s no ring at all.

Here’s what I already know
• The SIP trunk shows as “registered.”
• Inbound routes are set up and look correct.

Despite that, calls still fail to ring, so I’m clearly missing a detail somewhere (DID pattern, NAT, firewall or similar). I need someone who understands Grandstream UCM-series quirks to:

1. Remotely inspect the PBX configuration (trunk, inbound routes, extensions, firewall / NAT).
2. Pinpoint and resolve whatever is blocking the ring.
3. Provide a short summary of the fix so I can repeat it if needed.

Screen-sharing or a quick SSH/HTTP session is fine; I’ll supply access. Once your tweaks let the phones ring on incoming calls, the job is done.