I use a Polycom SoundStation IP 6000 with a configured SIP-server. My problem is I can call other phones but cannot receive calls with it. The Soundstation rings short time and displayes 1 more missed calls. The phone status is "Registered line" so it have to call.
My first guess is that the timeout on your phone system's dialplan is set to 1 second for this extension. In most cases, an issue like this is not actually an issue with the phone itself. I would be takeing a good look at that phone system.