Query for InstantMessaging b/w softphone in LAN via Asterisk

Hi all,

I have installed twinkle VoIP client properly on ubuntu.
I have installed and configured asterisk server and made accounts of twinkle clients on asterisk server so that they can contact each other via asterisk.

Both clients call to each other properly.

But, when i try to send message to another client,

after typing message, when i click on Send button,
it shows error on twinkle client : Delivery Failure : 405 : Method Not allowed.

But, message goes to asterisk server, and asterisk drops it.

chan_sip.c:9769 receive_message: Received message to <sip:123@192.168.5.8> from "789" <sip:789@192.168.5.8>;tag=lqptc, dropped it...
Content-Type:text/plain;charset=utf-8
Message: hello blah blah

Someone on twinkle forum told that asterisk is not proxy, its B2BUA and doesn't provide MESSAGE feature.
So, after searching on net, i found that asterisk uses SMS gateway to send messages to mobile phone using Bayham's FastSMS, ZimSMS applications.

But, i m trying to send messages between two softphones that are in LAN alongwith asterisk server.

Right now, i need to do check Instant messaging via asterisk server. I don't want to use any gateway on internet. If i want to do it in my private LAN then what should i do to redirect messages coming on asterisk server from one softphone to another softphone on another end of asterisk.
What applications is going to provide this message redirection between softphones via asterisk.

Please help,
I really need it.

Waiting for response.
Regards,
Tarun.