There is something amiss about AU->UK roaming calling. It hasn't worked properly for a couple of years, both myself and my wife have experienced it. But no one is interested in solving it, UK operators insist their services are fine (and generally they are everywhere else) the AU operators insist their services are fine (and generally they are using local sims).

Rock and hard place.

Should probably add SMS is fine, but calling (in or out) doesn't work.