Speaking of Sofia SIP support, I think that the http://sourceforge.net/projects/tp-sofiasip project is quite lively these days. Maybe we'll have this ported to maemo one day...

Were you able to make any progress on this? I'm seeing the same behavior on my N810 (Chinook) with Gizmo (4.0 v97) and freecall.com.

It seems like there's just one piece missing in order to use the tablet for free incoming & outgoing calls.

Here's what I've done:

1. Gotten a Gizmo account
2. Gotten a Grandcentral phone number, forwarded to my Gizmo SIP number
3. Confirmed that incoming calls to my GC number can be answered via Gizmo
4. Gotten a freecall.com account
5. Added the freecall.com as a "secondary account" within gizmo, using "sip.voiparound.com" as the server
6. Confirmed that logging into gizmo authenticates against both gizmo and freecall.com

Unfortunately, all attempts to place outgoing calls result in the same thing... the gizmo side "rings" once, but the receiving phone never gets the call, then gizmo disconnects.

forget adding freecalls as a secondary in gizmo.
add freecalls in the default comm apps for the Internet tablet and it will work.
I did this a while back.
but freecalls is not forever, I cannot use my freecalls account anymore to make calls.
seems I have to buy credit.
service sucked anyways.

best bet is to pay for skype pro, which is 36 dollars for a year.
and use that to make outgoing calls.

Whats wrong with using Grand Central to make outgoing calls (click to call)? It is free.

that works also.
but its just a hassle having to open up a web browser to place the call.
but I do use that also to make free calls.