asterisk users - Oct 2004 - Asterisk answering sipphone.com calls - stupid newbie question

Hi-

I've been playing around with asterisk (from CVS - very recent) for a 
few days on a NetBSD 2.0 system, and I was unable to get asterisk to 
"answer" an incoming call from sipphone.com until I placed the line 
"insecure=very" in its configuration.  Since the documentation on this
option is somewhat light, I'm not sure if this is the best way to solve 
my problem.


My config is like this.  In sip.conf, I have:

register => 17472345678:mypass@proxy01.sipphone.com/17472345678

[proxy01.sipphone.com]
type=friend
secret=mypass
username=17472345678
host=proxy01.sipphone.com
dtmfmode=inband
callerid="Jeff Rizzo <17472345678>"
qualify=no
reinvite=no
canreinvite=no
nat=yes
insecure=very

Without "insecure=very", asterisk seems to reject the incoming calls.
(I can dial out with or without that line)  Is there some other way to 
get asterisk to accept the calls?  What am I opening myself up to here?

Thanks in advance,
+j