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.
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'