"Hey Cortana" voice activation blocks "OK Google" on U.S. Android phones; feature removed in update
Activating "Hey Cortana" also caused problems with Android users unable to activate the "OK Google" hotkey. In addition, there were times when Google could not be activated even by pressing on the search bar or the microphone image. We can tell you that this is exactly what happened to our Motorola DROID Turbo. Once we deactivated "Hey Cortana," all functionality returned.
Besides removing the "Hey Cortana" voice activation from the app, the update improves the stability of Cortana for Android and adds enhanced call and text features. You can find the update in the Google Play Store by clicking on the hamburger app and clicking on My apps and games.
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source: Reddit via AndroidCentral