While there may be a way around it, I would think that for security reasons, given that 3CX really wasn't designed for this, you may be out of luck. As far as I'm aware (or can remember) an outside call is placed in one of 3 ways (all calls being linked to an extension number for billing purposes).
Normal origination, forwarded to an outside number, and dialled out though the voicemail menu( again ,of an extension).
So I think you'd have to have the calls appear to originate from an extension number.