Resolve PBX Desk Phone Registration

I recently migrated our system to a new PBX and I’m running into inconsistent desk-phone registration. Some handsets pop online right away, others refuse to register at all. The odd part is that every phone shares the same configuration profile; the only difference is location— the non-working units sit on separate sites/networks while the working ones sit locally.

Here’s what I need from you:
• Pinpoint why the remote desk phones fail to authenticate or stay registered.
• Verify SIP credentials, transport settings, NAT, firewall rules, and any VLAN or QoS factors that could block signaling or RTP.
• Adjust the PBX’s provisioning templates or create a site-specific profile if that proves more reliable.
• Walk me through any router, firewall, or SBC changes so I can replicate the fix at additional branches.

Acceptance will be simple: once every remote desk phone reaches “registered” status and can both make and receive test calls with stable audio, the job is done. I can provide SSH/HTTPS access to the PBX, phone web GUIs, and packet captures if needed. Let’s get those remaining phones back online.