If it is the older A800, you have to understand that this product is no longer manufactured by Quintum nor do they offer any support on it. Additionally, the SIP firmware that is provided for this unit is very old and just beta. It was never fully tested and again, there is no support for this. The story here is that it may work and it may not. If it does not work, there is not much that can be done as quintum will not fix it.
With that said, if initial registration works, but then fails later on, you should start to troubleshoot this. Start looking at the quintum sip logs (sproto) as well as your sip server logs to see what happens to the messages. Maybe quintum sends the message out, but your server never receives it. Maybe it receives it and responds but quintum does not receive it. Work the problem first to determine where the failure is at.