are prank calls illegal?

In MY State, yes.  There is also a case at trial now.  Someone "pranked him," said his daughter had been kidnapped.  Man called the Police.  When the Police got to the house the man had chest pains.  Police called ambulance.  Man died on way to hospital.

Two teens who went to school with the daughter (who was not with them, barely knew them from school, had refused to date one of them) were charged with manslaughter.

In my state they can be. We have a specific law titled "unlawful use of phone". It is a illegal if the call is done "with intent to harass, annoy or offend another person". The penalty is more severe if done "with intent to frighten, intimidate, threaten, abuse or harass another person with threats of violence" 

nah i used to do em