I tried to set up a prank once, involving a call from someone posing as a Secret Service agent who during a foreign-dignitary-protection-detail had an accidental firearms discharge and had killed the ambassador from [IIRC] France, and wanted to know what to do and if he needed a lawyer. I wrote the script, but couldn't find anyone who could both disguise their voice and go through the routine without laughing.

We also got our share of unfunny calls. My favorite story of these: around 1990, one of my superviors — then a rookie AUSA — was on phone duty. The FBI called and asked him to stand by to help them prepare very urgent search warrants. When he asked why, the FBI agent said that they had received a threat regarding a ex-Soviet suitcase nuke smuggled into Los Angeles, and that though they got many bogus threats of that type, this one concerned them because the technical description was so detailed and consistent with their intel on suitcase nukes. They said that they would call him back after their tech contacts at Livermore Labs got back to them.

Then they didn't call back.

It was an unpleasant weekend for my supervisor.