This site indicates that “crank” preceded “prank,” but gives no attribution. Anyone???
Google searches indicate a slight preference toward “prank,” 2.8M vs 2.4M.
A search of my newspaper database from 1940-1965 tends to favor “crank call” earlier and more often. “Prank call” seems to take over in the late 1960’s or so.
When I was growing up (1970s, South Carolina), we called them “crank calls.” I never heard “prank call” until much later.
It’s been rumored for years that Bell did not really need Watson.
Why “crank” in the first place? “Prank” is perfectly understandable. 
It’s not even clear to me that the two terms mean exactly the same thing. A crank call is a call made by a crank (i.e., an eccentric person). The implication is that such a call could only be made by someone who’s a little bit crazy. A prank call is one made as a prank (i.e., a mean joke played on someone). The implication is that such a call could only be made by somone with a nasty sense of humor. Who knows why the term “crank call” is being used less and the term “prank call” is being used more these days? Perhaps the term “crank” is just used less these days and the term “prank” is used more. Perhaps people have a different view of such calls these days.
Crank a random number of times get a random house on the party line? I dunno.
Admittedly it’s not even 10am yet, but this is the funniest thing I’ve read all day.
The more modern use of “crank” then describes someone that’s, well, cranky. The use of “crank” for a nutter seems rather old fashioned to me, but now that you’ve reminded me of that usage, it all makes sense now.
I remember when I was 10 yrs old getting corrected when I said “Crank Call” by one of my peers. Up to that point I had not heard “Prank Call”. This thread makes me feel slightly vindicated! 