I think they mean holding calls. I.e. you call 911 for a minor MVA, someone else calls for a shooting, another retail theft with the subject gone. Say 3 officers on duty in that area, all 3 will go to the shooting and the others are “holding” waiting for a available officer.
No idea idea the normal “holding” number is in ATL, I doubt is as anywhere near 400.
Please press 5 to be placed on hold. you are number 506. Your average wait time is 56 minutes. please hold. beep boop bop dee do dooo doo doo. please dial 911 if this is an emergency and press #FU12 for extra attention press #cake and eat it.