Lan calling

I’m having a problem where lan phones can not call each other they are cisco 7940’s. They boot up fine one has the extension 301 and the other has 300. here is what I put in the extensions.conf file

exten => 300,4,Hangup exten => 300,1,Answer exten => 300,3,Voicemail(44) exten => 300,2,Playback(tt-weasels)

In the sip.conf file I have


type=friend ; either “friend” (peer+user), “peer” or "user"
username=test ; usually matches the section title
fromuser=test ; overrides the callerid, e.g. required by FWD
callerid=test <300>
host= ; we have a static but private IP address
nat=yes ; there is not NAT between phone and Asterisk
canreinvite=yes ; allow RTP voice traffic to bypass Asterisk
dtmfmode=info ; either RFC2833 or INFO for the BudgeTone
mailbox=1234@default ; mailbox 1234 in voicemail context "default"
disallow=all ; need to disallow=all before we can use allow=
allow=ulaw ; Note: In user sections the order of codecs
; listed with allow= does NOT matter!
;allow=g723.1 ; Asterisk only supports g723.1 pass-thru!
;allow=g729 ; Pass-thru only unless g729 license obtained
astdb=chan2ext/SIP/grandstream1=1234 ; ensures an astDB entry exists[/code]

I have each for both line of course and when I try to dial the other one I get "Calling (out INV) which from what I read is a NAT problem, I’ve played around with NAT but still can not figure it out. Any help is much appreciated