belden...Are you fucking slow?! Why would you keep it in your leg pocket instead of your chest pocket, near the center of your body, aka your core, where your heart is, where your body keeps the warmest at all times.
Either way if you're skiing in -10 or even 10 degree weather none of the pockets in any of your clothing will be close enough to keep your phone battery warm enough. Hence why everyone is amazed at the toe warmer solution because it's simple and is available to everyone at any hill for 99 cents.
simple mate, your jacket is usually packed out underneath with a couple of thermals, fleece, wool, whatever, plus whatever insulation is in the jacket.
thus hot air stays trapped in the layers closest to your body, I found in -25 too -35 (western alberta) that my jacket pockets were simply not hot enough.
pants on the otherhand- 1 layer of wool, then ski pants, and as a result my leg pockets are much warmer.