There is the TW option to Stream Local Calls (or something like that). Enable that and the 2 sets will stream voice directly to each other, instead of relaying it through the TW server, only to go right back to the remote site. Seems like a waste of bandwidth to me. And saying that, since both sources will show up as the same IP to your firewall, it may be getting a little confused trying to stream that audio right back again.
In a nutshell, any local network where a phone will reside, whether it's on remote lan via Teleworker, or on the Internal lan with the 3300, all of those networks should be added as a Local Network in the MAS server.