Ignoring phase synchronization concerns, a cell phone can transmit about 3W of power, so a million of them would be about equivalent to the 3MW of the HAARP transmitter. Frequencies are different, HAARP is in the MHz range, cell phones are GHz, so while I bet your cell phone array would have less effect on the ionosphere, I also bet it it would cook a mean hot pocket.
Most current day cellphone antennas don't have phasing for any tuned directionality.
There are hard limits to array size due to physics and phones are just very small compared to yhe wavelengths they use.
And then there is the general challenge of synchronizing the transmitter phase. There is only so much that can be done via GNSS.
If cellphone frequencies were not filled with cellphones, it could make for a neat radiotelescope on receive.
Do users who unknowingly contributed sensor data get a sticker or badge or anything? Reminds me of seti@home.
What else could you do with full control of a global botnet of high powered sensor packs like android phones?