Researchers have identified dozens of apps on the Google Play Store that are able to display ads on top of the Android phone interface, a scheme known as AfterCall. When installed, they request overlay permissions, which allow them to appear on top of other apps—and thus display ads over phone calls. It's worth noting that overlay permissions can't be granted via a standard pop-up during the app installation process. Android apps are supposed to request these permissions in context and give a clear reason for doing so. If you are seeing ads pop up after phone calls on your Android, check for apps that have overlay permissions.