asterisk users - Jul 2005 - asterisk newbie and phones which don't want to comunicate

Hi list, i'm an asterisk newbie and i've to setup a net with an
asterisk server and several ip phones linked on the net.
i hope my questions are IT ans if you have some link for solving those
problems please mail me.
i've wrote the sip.conf in this way:
[2011]
type=friend
username=2011
secret=1945
regexten=1234
host=dynamic
;host=192.168.100.242
permit=192.168.100.0/24
context=sip-incoming
canreinvite=yes
dtmfmode=rfc2833
nat=1

[2012]
type=friend
username=2012
secret=1945
regexten=1234
host=dynamic
;host=192.168.100.221
permit=192.168.100.0/24
context=sip-incoming
canreinvite=yes
dtmfmode=rfc2833
nat=1

and the extension.conf if quitelly the same as the original. the
phones softwares are setted up correctly, but from a phone i can't
call another phone on the net. can somebody suggest me a possible
solution?

thanks a lot