Just to let you guys know;

I was receiving and sending choppy SIP audio (also choppy Skype, GTalk) using wifi and 3G connections, the choppiness wasn't permanent though.
It turned out the amount of bandwidth available on my 3G connection was quite small (especially upload), to small for most codecs. And my router didn't prioritize VOIP traffic (Torrent traffic was interfering with VOIP traffic). I solved it quite easy by:

1) Configuring my sip account so it only uses iLBC, (it seems this codec uses the least bandwidth, that's why I chose it).
2) Did some QoS configuring in my router (WRT54GL -- Tomato
Version 1.27) in order to give SIP (and other VOIP services) absolute priority over all other traffic.

These two little tweaks did the trick and SIP/VOIP is working like a charm now.