suggestions for keeping phone warm?

having issues keeping my phone warm once I hit the hour mark on an outdoor ride. any suggestions i should try? i keep it in my zip pocket on my chest under a vest and its still ice cold even after 20 minutes. i have tried a chemical handwarmer but it didn't seem to help much. we are talking -20c conditions thereabout. is this just wishful thinking or do i just need a new non shitty phone ha