is it dependent upon configuration?
It surely is. And the configuration will depend on your requirements. If you do recoding, call recording or if your external users are behind a NAT or unable to contact your SIP provider directly, you can't bypass the PBX in the media path.
The asterisk extension option for a direct media communications path is either "canreinvite" or "directmedia" depending on the version. It has a couple of prerequisites to really work as intended - refer to the docs for details.
If you have external users in external networks you are not in control of, I would suggest not assuming that a bypass mode would work when planning your infrastructure requirements.