Masum,
first, the A800 really does not support sip. I know that Quintum has some beta/test software on their web site for the A800, but my sources tell me to stay away from it as it does not work very well.
As for the AX2400, there are some docs on Quintum’s web site that discuss SIP and this may help. If you want me to provide you the information, you can contact me at mike_voip@hotmail.com to discuss my fees for providing this service.
Mike