Problem Description:

We have a customer who encountered this problem: after we connected 3CX IPPBX with Voptech FXO gateway, when we dialed FXO number from outside, we didn’t hear the welcome voice from 3CX IPPBX side or the specified extension didn’t ring. We have double checked the settings, but no problem was found.

Problem Analysis:

In the current scenario, the 3CX IPPBX and FXO gateway are both in the same local network, in order to check if the incoming call is transferred to the 3CX IPPBX, we capture the packet at the 3CX IPPBX when calling in from the FXO gateway, and then open it with wireshark and find that a call arrives at the IPPBX, but 3CX IPPBX firstly sends 407 to request authentication of the call, and the authentication fails and then replies with a 403 error.

Because FXO gateway connects with 3CX is through IP to IP, not registration, so under normal circumstances, 3CX will not make a 407 authentication to the call sent by the gateway.

To find out why 3CX IPPBX authenticates it, we open the first invite request received by 3CX IPPBX:

The 8001 in contact is same as the digital receptionist number in 3CX server, and it could be that the 3CX server sees it as its own extension, so 3CX server will authenticate the call.


The problem disappeared after the FXO number was later changed to another number at the gateway: