How to display the original Caller ID when call forwarding through Sipgate sip trunk

[Issue Description]

When an incoming call to PBX's extension and the extension set call forwarding to an external number, at the external side can't see the original Caller Number. 

[Cause]

Configuration issue

[Solution]

First of all, please contact the SIP provider to get the correct configurations. Then please configure the PBX as below. 

Go to the SIP trunk page->SIP Headers->Outbound Parameters

1. Set the 'From User Part' with 'Trunk Username',

2. Set the 'From Display Name Part' with 'Originator Caller ID Name'

3. Set the 'P-Preferred-Identity' with 'Originator Caller ID'