Can we do direct SIP call?

Test using EXACTLY two parties
Source:
Direct calls between two SIP phones without passing through a SIP proxy
www.switzernet.com/people/emin-gabrielyan/070405-budge-tone-without-sip-server/

You will need to assign a different SIP port to each phone and some RTP ports need to be available. Assuming Aastra phones allow IP to IP calls you should then be able to go ahead an call the other phone by IP if that is what you want.
Source:
Connecting two SIP phones without a SIP server
www.voipuser.org/forum_topic_20099.html

As long as you can set Enable IP Dial: to Yes in the Admin/Advanced page of the SPA, you can
Source:
Direct IP dialing with Sipura without SIP server?
www.dslreports.com/forum/r9100980-Equipment-Direct-IP-dialing-with-Sipura-without-SIP-server-

SJphone (http://www.sjlabs.com/sjp.html ) does support direct IP calls.
It appears that X-Lite supports direct IP
calling. Just do NOT register with server, make sure both hosts allow
incoming packets on UDP 5060-5061 so they can see each other, and
create accounts so you can dial "test@remoteip".
Source:
Direct IP call with X-Lite 3?
www.velocityreviews.com/forums/t392916-direct-ip-call-with-x-lite-3-a.html