I have already re-installed the google app, factory reset my phone, but I still can't find more options fron the 'Ok Google detection' page.
This is what I see:
Click image for larger version
My Phone: Sony Xperia C3 Dual
Android version: 5.1.1 (lollipop)
Not all phones support hotword detection being on all the time (i.e. "OK Google" from any screen). The phone needs a lower-power co-processor, to avoid running down the battery by keeping the processor running at full power all the time just processing sounds. It's up to the system integrator (the people who compile Android for the phone) to set whether this option is enabled or not. From other users' reports on Google Product Forums, it looks like the Xperia C3 doesn't support this, and Sony removed it in the update to Lollipop.
Most likely you could get this working with a custom ROM, but the cost would be that it would run down your battery - which is presumably why Sony removed the functionality in the first place.
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