[Asterisk-Users] SIP signaling vs Media (Voice) Traffic

[hugolivude]

I have an Asterisk serving 15 people using the X-Lite soft-phone. 
Currently they all register to the internal IP address of Asterisk
(192.168.1.110).  I only use VoIP internally. External calls go PSTN.

I'd like to arrange it so that they register to our external WAN
address (port forwarded to Asterisk) so that they can go mobile and
still have Asterisk service.

Is it possible to arrange it so that when in the office, the SIP
signaling goes through the external WAN, but the Media (Voice) traffic
stays local?  In other words when a user is on the local LAN, I don't
want their voice traffic going out on the net and then back in.

Thanks,
Hugh