I'm on a similar quest to set up VOIP on my N810. I'm still doing the background research and haven't tried to get it working yet.

If you want to use the standard SIP phone software that comes installed on the table, my impression is that you need to sign up with a VOIP provider like voipbuster.com. Note that you will NOT need to download their software onto your N810, you can use the SIP client already on your tablet. If you want to receive a VOIP call on your PC, you will need VOIP software. And my impression is that x-lite from counterpath.com is a reasonable choice.

The other thing you could consider is using Skype instead of SIP, since they have software for the N810 that you could download and use.