Just a tip for anyone tring to use the tablet as a SIP phone with the built-in internet call app. Check the call settings (Tools-->Settings) and ensure 'Receive calls from" drop box is set to 'All'. I had a similar problem to the OP when I first started using VoIP on the tablet and was stumped why I could not call my tablet. You can also set it to 'none' or 'known contacts'. If you set it to 'none', the Internet Call app will give you no indication that a call is coming through.