Although this question pertains to a movie I think there should be a factual answer.
In the movie “The Maltese Falcon”, Sam Spade (Humphrey Bogart) tells Brigid O’Shaughnessy (Mary Astor) that he will signal her by ringing her phone with a series of 4 rings, long then short, followed by long then short. The movie takes place in the early 1940’s.
How could someone change the length of a phone ring, or was this just poetic license?
Let the phone ring longer for the long 'ring ’ , I use to signal a friend by letting the phone ring 2 times hang up then call back and let it ring longer . We didn’t caller ID then and this way my friend knew who was calling her.
On old-style hand-cranked telephones, turning the handle caused all bells on the same line to ring. So, you could just crank it in the cadence you wanted.
Yes OP, Bogart was talking about changing the number of rings–not the length of each ring. And you change the number of rings simply by changing how long you keep the phone off the hook after dialing before hanging it up.