The short answer
A phone that will not register is one of two different problems, and the first question is whether it ever ran on our system.
A phone that never ran here
This is a setup case, not a fault. Three things have to be true: the phone's MAC address is on an extension on your account, it was added under the right family (an Edge or a VVX under Polycom BusinessMedia, a Yealink under Yealink; the wrong family never registers), and the phone has been pointed at our server once, which is the single manual step in setting up a Poly phone or setting up a Yealink phone. A phone that boots and shows the provisioning screen but pulls nothing is usually missing the MAC or on the wrong family. Give it five minutes after any change; a fresh phone reboots more than once.
A phone that used to work
Unplug it and plug it back in first; after a router change every phone needs that. Then read the phone's own Status pill on its extension's Phones tab in the portal. It says Registered or Not Registered, and it is built from the live registration, so it is the one indicator that answers whether the system will ring that phone. Two other lights look like the same answer and are not: the green dot next to the phone means it checked in for configuration within the last day, so a phone that died an hour ago still shows green, and the "did this phone reach our server" panel reports provisioning traffic, not registration. A Poly screen that says "URL Call Disabled" when you dial is its own wording for Not Registered.
Read together: recent provisioning traffic with no registration means the phone is on the network and the registration is failing; no traffic at all means the phone is offline, unplugged, or its internet is down.
The test that settles it
A phone showing an IP address is not proof of a working network. Move the phone to the network jack of a phone that works. If it comes up there, the problem is that jack or that part of your network, not the phone and not us, and your own IT takes it from there. It needs no router access and it settles the question faster than anything else.
Firewalls
Phones reach us outbound only, so nobody has to open a port in your firewall. When every phone at one location fails and phones elsewhere are fine, the location's firewall or filter has changed; the ask to your IT is to allow outbound traffic to our provisioning and calling servers, and we send the exact hostnames and ports to your network person on request. We do not hold router credentials, supply routers, or configure your network.
A factory-reset or second-hand phone
It may hold another provider's provisioning address. Send us the MAC address from the back sticker and we re-provision it remotely; there is nothing to type on the phone. The one exception is a Yealink being set up for the first time, where entering the address is a normal step. A phone locked by a previous provider needs their factory reset first.
Wi-Fi phones
A Poly Edge E350 on Wi-Fi needs a 5 GHz network whose name has no spaces. It does not join 2.4 GHz networks.
When to call
With the MAC address, the model, and what the screen says. We read the provisioning log for that phone and see exactly what it asked for and what it got.