What does the phone icon mean on chat ( hear me out )?

[J Blitzer]

I know it means that you are on a mobile device, but whenever i am on a mobile device it shows me as a green circle. Never the phone icon. I seen people active on Facebook for hours with just the phone icon by their name ( without the time ).

[Daniel]

When your Not using the Facebook App And Close it out Facebook Detects that you have Closed it so The Green Icon Turns into the Phone Icon After a Certain Amount of Time the Active Minutes Stop Displaying