Oreo doesn't use numbers for badging. A badge just means that there's a notification if you swipe down. (If you clear all, even though you haven't read anything, the dots go away - they're "there's a notification from this app for you", not "there's a call [or text or whatever] for you".)
It's part of Oreo. If you want the old-style "number of calls" badge, try Nova Launcher with TeslaUnread for Nova Launcher, or some other launcher that does the old style badging.