Shared through the game's official Twitter account, Rocksteady posted a US phone number: (310) 564-7047. If you call it, that'll play an automated message from Amanda Waller, the Suicide Squad's fictional handler.
Suicide Squad: Kill the Justice League ARG Lets You Prank Call Amanda Waller
Assuming that you're Harley Quinn playing a prank, you'll get a brief but heated message, stating:
I know this is you, Harley. I have given you weapons, resources, and full sign-off to execute the Justice League and you’re here spending my time on prank calls? Stop screwing around and take the targets down. Now!
You're the one who asked for a chance to prove yourself Shark, and that's what I'm giving you. I don't care how you kill the Justice League: stab 'em, shoot 'em, bite their shiny little superhero heads off. You're not going free until we're done.
It's a fun little promotion in the run-up to DC FanDome 2021, and we'll keep you informed of any new Suicide Squad: Kill the Justice League reveals as they happen.