Orientation wouldn't be that hard an issue to overcome, phones have magnetometers to act as a compass, and accelerometers which indicate the direction of gravit

Orientation wouldn't be that hard an issue to overcome, phones have magnetometers to act as a compass, and accelerometers which indicate the direction of gravity, so assuming you have a decent idea of where on earth the phone is located, you can work out its orientation in 3 space. Precision can to some degree be made up for by quantity. If data from all the phones in an area skew the same way, it can stand out even if the signal would be impossibly lost in the noise from a single detector. More problematic would be calibration. A phone at the top of a tall building vibrating like a tuning fork is going to have a radically different signature from one at the same altitude but in the pocket of a mountain climber, which in turn is going to be very different from one at sea level but close to a highway. If you had a sensor sitting in the same place for a long time, you could probably characterize it, but assuming these are phones people are going to pick up and move after a couple minutes I doubt you could do more than a very crude guess.