This is most likely some device-specific Android setting.
We recommend doing a search like push issues on the device.
There is also an app that can sometimes help with this.
In general, battery-saving/security features are the primary cause of device-specific issues with push on android.
You (or the end-user) should go through the device settings and disable anything that might relate to battery saving, limiting background use, or limiting network access, at least for your app.
If you or the user still have issues after this, please open a ticket for the issue in the cloud softphone portal under your app with a description of the issue and exact steps on what you have tried to fix the issue and we will take a closer look.