Not funny at all, is the answer, unless it is done to family and friends who are able to understand the 'in joke'. When it is done to a stranger, it is just cruel because they are unsuspecting 'victims' who are not in on the humour that is being enjoyed by the perpetrators. Also, the balance of power is in favour of the person carrying out the joke. The 'victim' is powerless. What is funny about this? Prank calling is what children do for fun till they grow out of it. Grown up adults doing it as part of their job is shockingly immoral and juvenile.
There is a huge amount of anger and hate being displayed against the Australian radio presenters who carried out the prank call that caused the death of the nurse. Some will say it didn't 'cause' her death but, given the timing, it must have been a cause. I can only assume as many others do that Jacintha was embarrassed that she had let the prank callers talk to the Duchess of Cambridge's nurse.
It is absolutely wrong that people would find a prank call to a hospital funny when the Duchess had a serious condition. The fact that the presenters endangered two people's jobs should have showed them that what they were doing was completely immoral. Jacintha Saldanha allegedly took her life over something very petty that should not have happened. I pray that she is resting and happy.