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At least on recent iPhones, the front-facing camera does not have IR filtering, but the back one does.

Source: I use the front camera to debug one of our products, which uses IR LEDs as a flash to check for obstructions.

I thought most phones had IR filters these days and that this wouldn't work? Am I incorrect here?

I just checked mine now, new Moto Play Z so pretty recent, and it's much less noticable than it used to be. However, going into a dark spot so nothing else could be seen, the led in the remote flashed a slight purple. Definitely enough to check it's working, but you need a bit more cover around it to be able to see it. In the past, they used to flash like bright bulbs.

They have basic IR "filters" which are used to reduce IR light, but they don't totally block it, which is why you can see a purple light coming from TV remotes and such [0]

0: http://www.wikihow.com/images/6/60/Check-if-a-Remote-Control...

The cell phone camera is a great way to test whether a multimode fiber optic cable is connected at the far end, too.