In version 14 and before there used to be a '302 diversion header' checkbox in 'Away' and 'Out of office / DND' forwarding profiles. Unfortunately this feature is gone in version 15. It allowed, if supported by telecom provider, to divert the call to new destination keeping original Caller-ID without using channels on the SIP trunk (otherwise you occupy 2 channels -- one for incoming call to 3CX and another for outgoing call to destination). You could name this method 'Forward via carrier / telecom provider' and was also found in legacy (TDM) PBXs for analog / ISDN trunks.
If you forward the call via 3CX, you will use 2 channels from your SIP trunk(s), eventually you can send original caller-ID, if supported by outgoing call telecom provider, by modifying outgoing parameters of the trunk (sending originator's caller-ID). Otherwise you will send intermediate caller-ID of the extension called and forwarded to external destination or trunk's caller-ID.
In Asterisk, nevertheless not so trivial, you can program usage of the diversion header, which allows you even to transfer call to external destination, after being answered, keeping original caller-ID via telecom provider, i.e. without using SIP trunk channels during duration of the transferred call.
