A phone riddle is:
 
Q: What kind of phone makes music?
A: A saxophone.
 
The joke has been cited in print since at least 1976.
 
       
Google Books
Biggest Riddle Book in the World
By Joseph Rosenbloom
Illustrations by Joyce Behr
New York, NY: Sterling Publishing Company, Inc.
1976
Pg. 132:
What kind of phone makes music?
A saxophone.
 
26 October 1980, Newsday (Long Island, NY), “Smiles,” Kidsday sec., pg. K9, col. 3:
What kind of phone makes music?
A saxophone.
—Evelyn Bialowitz, Douglaston
     
Google Books 
Swine Lake:
Music & Dance Riddles
By Charles Keller
Illustrated by Gregory Filling
Englewood Cliffs, NJ: Prentice-Hall
1985
Pg. ?:
What kind of phone makes music?
A saxophone.
 
Google Groups: rec.humor
Q&As
Nancy Carson
11/11/95
What kind of phone can make music?
A saxophone.
   
Google Groups: fido7.su.humor
Ha Ha Ha
Alexander Novikov
9/13/98
(...)
- Heard in the Lunch Room -
What kind of phone can make music?
A saxophone.
       
Google Books
The World’s Most Crazy, Wacky, & Goofy Good Clean Jokes for Kids
By Bob Phillips
New York, NY: Galahad Books
1999
Pg. 202:
Edna: What kind of phone can make music?
Eldon: I have no idea.
Edna: A saxophone.
 
Google Books
Sensible Nonsense.. Frank’s Comic Pack:
Humour Pack Series
By Frank Paul
BookBaby
2014
Pg. ?:
What kind of phone makes music?A saxophone.
 
Twitter
Lucky Score
@LuckyScoreNet
What kind of phone makes music?
A
saxophone.
#Joke from #TelephoneJokes in #Jokes, #Humor and #Satire
11:07 PM - 19 Apr 2018