What's your 20?

[Sarah]

The phrase essentially means, What is your location? or Identify your position, but is a corrupted phrase from the original 10-20 used by United States law enforcement to verbally encode their radio transmissions to that non-police listeners would not easily discover police operations, as well as to communicate quicker and more efficiently by standardizing frequently used phrases.
These verbally-coded messages were called 10 codes, of which 10-20 stood for Identify your position, or Where are you? originally. Other such codes include 10-7 meaning the officer was busy such as with a traffic pull-over, 10-8 meaning that the officer was back on patrol such as from having just written a citation, the popular 10-4 as an affirmative, 10-10 as a negative and 10-22 to disregard a previous transmission have only seen light integration into common use. It was not uncommon for a city to have its own set of particular 10-codes for other phrases frequently used particular to that locale.
This code-phrasing is similar in design to Amateur Radio Operators (which require an FCC license) use of Q-signals, such as QTH (What is your location) and QSL (affirmative/understood) used to reduce the time needed to transmit and interpret a Morse-code transmission.