Some of the Chinese phone manufacturers have very aggressive power management policies to maximize the battery life, however this leads to lack of notifications.
Below you can find the 5 steps that will help you restore the PUSH notifications on your Xiaomi device with MIUI, so you can keep receiving calls with your Zoiper:
Note, step number 5 is recommended only for advanced users which know what they are doing and on their own risk!
Step 1: Go to settings – manage apps' battery usage – choose apps. From there, choose "Zoiper" and select - “No restrictions.”
Step 2: Go to settings – permissions – autostart. From there, choose Zoiper, and toggle the switch to turn it on.
Step 3: Go to settings – app notifications – choose again Zoiper, then turn the toggle for “Priority” on.
Step 4: Lock the app in the “recent apps”/ “app overview” plane. Do so by first opening the apps, then press the “recent apps/overview button” (that’s the square button on stock Android or the button with three horizontal lines on the Mi Mix or swipe up and hold if you’re on MIUI 10). From there, find Zoiper, pull down on it to “lock it”, so it never gets cleared.
Step 5 (at your own risk): This last step requires Developer Options privileges. To enable that, go to settings – About phone. Tap on MIUI version tab like eight times. You should then get a little message saying “you are now a developer.” Then head back out to settings, go to developer option, scroll to nearly the bottom, find “memory optimization,” and turn it off.