I have a Lumia 640 with T-Mobile.
Normally on 8.1 update 2 that came with it, you have to go to your settings to enable it.
Right now I have installed the latest Win 10 mobile update. In the settings, it shows up, but so far I haven't been able to get it to work. I am giving the Build some more time, otherwise I might go back to 8.1 again.