Go to the Stephan Leonoudakis Golden Gate Ferry Terminal behind the San Francisco Ferry Building. There you will find two pay telephones.
Some telephone keypads send their signal via Dual-tone multi-frequency. Each row of buttons on the keypad has a sound frequency associated with it; so does each column. When the user presses a key, the keypad combines the sound frequency of the row with that of the column to generate a sound wave.
Combining sound waves is hard. What if we just added frequency numbers instead?
Suppose these payphones' telephone numbers had "frequencies"
24 29 7 21 15 21 21 and
24 29 7 21 9 15 7
...then what would these frequencies mean:
21 24 6 9 24 12 14
(Not a clue, just a suggestion: If you're tired and hungry, there are food and drinks in the Ferry Building. If you can't find a place to sit in the Ferry Building, you might wander over to the Stephan Leonoudakis Golden Gate Ferry Terminal and see if there's an empty bench upstairs.)