asterisk users - May 2005 - A newbie question - SIP to Trunk

Hello,

Firstly sorry for covering old ground - I'm new to this. . . .

I've read that you have to be careful when configuring SIP phone extensions
so that an incoming call can't be connected to the trunk.
Anyone have some info on how this can happen and how to stop it?

Next,
Can anyone tell me (in outline) how to set up a wifi SIP phone so that when
I'm in the office I call in/out over Asterisk and the trunk and when I go
home I can still be called from the office and still use the office Asterisk
for trunk calls.
Of course the office Asterisk is behind a NAT/firewall.

Thanks in advance.