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?

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...

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.

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