The Capone character made the one-handed telephone gesture with his thumb and pinky. He then answered a telephone call on a candlestick phone.
Oops.
Good catch.
According to this site, the combined speaker/receiver phone was around at the time, but wasn’t available on the Bell system till the late 20’s. So its pretty unlikely people in the States would’ve understood the thumb/pinky “call me” sign in the early 20’s.