Though mobile voip is certainly becoming reality, you'll still likely
encounter issues with sound quality... That's the last piece of the
puzzle that is still ruining the "call experience" after all else has
been pretty much sorted out.

Given this little problem, I would recommend using some of the "semi-
voip" or hybrid solutions that combine a regular national phone call
and a cheap international voip call into a viable alternative against
regular international calls.

There are a few such services out there, like Rebtel and Jajah Direct,
but my personal favourite is Manifone.

They'll give you a few London numbers which you can "point" to
International destinations of your choice, which makes your
international destinations become available on UK numbers, at nearly
voip rates. Since your voice leaves your mobile handset in form of a
regular GSM call, you won't need a data plan or SIP client to use it.

I know that only true voip on your mobile will give you complete
freedom of choice, but technically it's not quite there yet. In the
meantime, use a hybrid alternative!