Hi Herbie,
I checked what you found, it's unrelated to your issue & it also applies to a much older firmware so I would not take that into consideration.
- You can ping the phone: this only proves that the phone can be pinged, it does not imply that everything else will work (ie. an IP conflict would still get you ping replies, but not necessarily from the phone). You cannot be 100% confident about the network, just from pinging alone - you need more evidence.
- You said earlier that you cannot connect to the phone's web interface, even though you factory reset it. This might indicate that either the phone has an issue, or it is a combination of your pc+network+phone that does not allow you to connect. This is something you will have to troubleshoot yourself, as we cannot look into your network for you
With all the information you gave us so far, the very least I can conclude is that the phone cannot be reached, and hence it's reasonable to expect that the phone cannot reach the PBX either. The reason remains unknown at this moment, so do not be quick to dismiss things such as the network, the most vital part of this chain.
What can you do?
1. Port Check: You said you have 80 phones and most of them work except 5. Unplug one that works, and in its place put one that doesn't work. See if it suddenly becomes accessible and provisionable once you use the LAN port and cable of the one that works.
2. Reverse Port Check: Do the reverse of the above test: Put the good phone in the port and cable of one that has the problem, and see if it also develops this issue. Do it even if test 1. has failed - don't skip this
3. Network Health Check: can it supply enough PoE power? Are your network/VLAN settings OK? are there enough free IPs in the pool? Did you try restarting the switch and firewall just for good measure? Was there any recent changes by you or colleagues on any network cabling, equipment settings, rules or firmware? All these details may help your investigation, even if they seem like small details.
4. Phone Health Check: Take a problematic phone outside of your network, and try connecting it there (at home for example). Can you access the web interface when you have the phone at another location? If yes - then you can be very certain that the phone is healthy
I hope this small guide is useful to you, go ahead and try these things out and let us know what you find at each of these 4 steps.
