I don't believe I'm doing this...
The power output of cell phones is limited to 2 watts:
http://en.wikipedia.org/wiki/Mobile_phone_radiation_and_health
Popcorn must reach 232 degrees celcius to pop.
http://en.wikipedia.org/wiki/Popcorn
I don't know what a cork kernel weighs, so I'll assume 1 gram and that it's mostly water (way generous estimate!!!!).
Heating this fatass popcorn kernel from room temperature to 232 degrees requires 212 calories or 890 joules.
http://en.wikipedia.org/wiki/Calorie
If 4 cell phones were set up with some kind of focusing dish 100% efficiently directing their power at the popcorn kernel, and the popcorn kernel were suspended in a perfect vacuum so it couldn't lose heat to its environment the process would take 111 seconds.
The corn in the video pops in about 20 seconds.