Mobile Device info showing 'not set'

I integrated google analytics with my android app. It's gives very useful information i need, but in Mobile Device info tab, i found some devices are shown as 'not set'. What it means? Am I want to configure any settings to get those data. Please suggest me the solution.

Change the 'primary dimension' to Operating system on the devices screen to get more visibility into 'not set' devices

ref: https://productforums.google.com/forum/#!topic/analytics/n-LDtWGhZiY

I'm from Turkey, and in my case, these not set devices are the devices that are produced by top ISP's or mobile network operators, thus, their information can't be collected by GA.