Yealink, provisioning and Sophos firewall

[complex1]

Hi,

I think you may definitely say the Sophos firewall is causing the issue, because a phone at your home, and behind the router firewall, is working well.
Somehow, the phones behind the Sophos firewall can not communicate with the PBX or VoIP Provider.
In the Sophos firewall, you need to open and forward mostly UDP ports to the local IP address of your phone.
Port 5060 UDP is the most important one.
I do not know which ports your provisioning or yealink servers uses.