It's not available on the Z and Z1 which are flagship phones, you can contact your Local support but there has not been an official statement as to why it's not available with other devices.
with that said, there are third party apps with that feature.
That's why I prefer to use built-in functions, rather than 3rd party apps. Pretty shame that this function doesn't exist on mid range phones. The Lumia phones has it, whole range.
