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.
B: I'm at a red light that won't turn green even though there's no cross-traffic.
A: What's your 20?
B: Avenue F and Kingston.
What's Your 20?: Weather better fuckin stay nice all fuckin day cuz once I leave this fuckin shit hole I gotta pick up my bitch daughter and her fuckin cunt kids.
Me: (Awkward smile and nod)
Whats your 20?: I tell ya, I fuckin ate some bean-picker food last night and, fuck, I've been shitting every 10 minutes since. The fuckin women's toilet looks like a fuckin diarrhea-shit grenade fuckin went off in there. Any chance I can bum a fuckin smoke off ya?
Me: (Blank stare)