I am looking at this menu here.
http://www.voipsupply.com/blog/wp-co...009/06/a52.jpg
Pay no attention to the other settings, they have nothing to do with Voxalot. It is just a picture I grabbed, since I do not own a Siemens A580IP
Play around with the registration refresh time. I would start at 120 seconds, and see what happens.