I think this is possible.
popcorn pops due to the moisture in it heating up and pops through the build up of pressure.
when mobile phone rings I think it can supply upto 3.5 watt of power. since the phone rang for 3 seconds there's about 9 joule of power. and there are 4 phones so about 36 joule of power.
so moisture is just water, the heat capacity of water is around 4.1813 which means 4 joules to heat up a gram of water by 1 degrees. Since heat capacity is an extensive property so it will be significantly less to heat up the water in popcorn which would be in the amount of miligrams or less.
If you do some shitty math it takes about 0.6 joule to heat up a miligram of water to 150 degrees. 150 degrees is needed to builed up enough pressure from steam to pop.
Thus it might be possible to pop it with 4 phones providing 36 joules in 3 seconds.
The only thing that's amazing is that the heat/energy was transfered using radiation. Radiation can transfer energy even in a vacum so the key was probably there was no energy loss from the phone to the popcorn, maybe.
