Consider the following scenario:
You are enabled for Enterprise Voice.
You use the Skype for Business app on an iOS-based or Android-based mobile phone.
You set Voice Settings to Cellular or VoIP over Wi-Fi only.
In this scenario, when you join a Skype for Business meeting, the Voice Settings value is intermittently reset to VoIP always.
The issue occurs because user's voice policy assignment is reset after the Skype for Business app is brought back from background upon receiving an incoming call.
To fix this issue, install the July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Web Components Server.