I have an inbound number of voipbuster. Outgoing calls go well via Voipbuster using the N810 builtin 'internet call' but incoming calls are not detected, no ringing or no activity on the display. I hear just a calling tone on the other phone in which I dial my Voipbuster number.
Has somebody similar problems ?

UPDATE:

I tried again and now it works ! Probably I dialed the wrong number ..???

Well, I have a weirder one: I do see the calls (from another sip provider, not voipbuster in this case), but as soon as I try to take them, they're immediately disconnected.
If I'm the one initiating the call, everything works as expected.

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.