Model: P-Series Appliance & Software Edition
Firmware: Not Required
In a general network topology where PBX and IP phone are in the same local internet subnet, if IP phone registers extension by PnP(In the Office) provisioning method, afterwards if you access web interface of pbx by FQDN domain, you will be possible to remote access web interface of the phone as well through auto provision, this feature benefits device supplier who remotely manages PBX and phones for end users.
There are some particular situations where pbx and IP phone are not in same subnet, which might cause unavailability for 'remote access' option, and furthermore you might notice there is no available IP Address for the phone in auto provision list, which actually will cause unavailability for 'remote access' option.
It is known that p-series detects ip address of the phone by SUBSCRIBE packet sent from the phone after the phone reboot, a typical principle is the SUBSCRIBE packet only possible to be broadcast over local area network(LAN), so if there is complex network topology which makes broadcast unreachable to pbx, it will not display ip address for the phones in auto provision list, therefore remote access feature for the phone will not be available.
Below shares some network deployment which would make SUBSCRIBE packet unreachable to pbx.
1) Local network but different subnet which blocks broadcast packet from phone to pbx
2) VLAN topology which blocks broadcast packet from phone to pbx
3) Auto provision the phone by FQDN method
4) VPN network between pbx and phone
With above network topology, 'remote access' option for the phone will not be available.
Why extension can be registered and can be reprovisioned even no available ip address for the phone?
It's possible manually to register extension on the phone or input provisioning link on the phone. After successfully register the extension, pbx will save the registered ip of the phone and later if some change is made for the phone, pbx will send new configurations to the registered ip to update the phone.
If pbx and phone are in different sites with different public ip, it's possible to register extension but not possible to remote access the phone, only if pbx and the phone are in same network subnet and broadcast packet can be well reaching pbx, thus it will be possible to remote access the phone from auto provision.