Goal
Create a long-running background service with a network connection, similar to Zello app.
Problem
Starting with API level 26 (Oreo), there're tight restrictions on background services and their network activity.
Looking at the Zello app which has a constantly running background service which is able to accept audio and text messages even if the device is sleeping, I wonder how they achieved that? Their service is not running on the foreground. Also, it doesn't look like they use push messages for that, since the app works quite stable in conditions where there's a problem with push messages reception (e.g. low-end Xiaomi phones).
Any ideas would be appreciated.