To add my two cents, I long quit any hope using SIP. It was useless : too choppy to even speak a few words : I could hear perfectly remote side but I wouldn't be understood. Skype has always been fine though, even on 3G, so I suspected my SIP provider Wengo (a service from French ISP SFR) codec to be N900 unfriendly.

Lately I tried to investigate further. I run CSSU with kernel-power. I read that SIP choppy problems could occur with that specific kernel.

But before reverting to Nokia kernel I remembered I didn't do any kernel upgrades with my recent PR upgrades

So I gave reinstall a go with :
apt-get install --reinstall kernel-power kernel-power-modules kernel-power-flasher

And ... voilà !!!!!! I can use SIP again through wifi. At least this morning I was able to have a loud and clear 10 minutes conversation before restarting the call. For me it's much better than nothing.

I also played with pulseaudio in /etc/pulse/daemon.conf going from 2 to 5 in default-fragments. Don't know if it helped.

Still for me SIP over 3G is very choppy, but this can be for a lot of reasons : codec is too bandwidth consuming for my weak 3G connexion, but I mainly suspect my provider to downgrade sip port (SIP over 3g is not allowed here). I should run some further tests. If anyone is interested, just let me know.