The two announcers wouldn't have made the decision to make the prank call; that would have come from their producer. And given the sorry record of the station, particularly in relation to the various incidents involving Kyle Sandilands prior to this, there can be little doubt that they have created a particularly ugly culture where the lowest possible denominator rules. This was probably the least offensive incident, until the death of the nurse.
The announcers will be sacrificed if it's going to save the station's bacon, but even though they're a pretty vacuous pair, the responsibility lies with the station - their choices, their culture, their methods.
But I also think responsibility lies with the hospital it's ridiculous that anyone can phone up and get a patient's records any patient, not just a royal. I'm still amazed that this doesn't appear to have been drummed into the staff as a matter of course. They're trying to shuck the blame onto the Aussies, but it's a smoke screen to cover their own guilt.