User presence and SIP

Hi,
I have developed an application that permits to start/receive a voice
conversation with Windows Messenger 5.1, using Java Media Framework and
SIP, both connecting to a local SIP Server. Everything works fine
except I can not make the online user presence work in Messenger as it
always says my application is offline. I am only sending a SIP
SUBSCRIBE message when I get online (and checking for Ok response), and
responding to NOTIFY requests with OK. What else do I need to do make
the user show online in Messenger?
I read that a SIP Presence Server would take care of this, but I am not
aware of an open source server with this capacity and tested a
commercial one (CommuniGatePro) that apparently doesn't solve anything.
Any help appreciated.
Sincerely,
Araxes Tharsis

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