The web interface of 3CX 15 is a bit different than version 14 but the configuration of a beroNet gateway with the IPBX is very similar.
Find out the IP of your Gateway
First things first: you need to find out the IP address of your gateway that you plugged to your LAN. To do so, run bfdetect. It enables you to find all beroNet devices on your network.
You can also use bfdetect to set a fixed IP address on your gateway (which is recommended).
Add a SIP trunk in 3CX
On the left part of the 3CX web interface, click on "SIP Trunks".
