[alandarev]

Oh, that is a fair point.

Though, not receiving any information back, draws the tracking practice significantly more difficult.

[Theodores]

It need only reply if it is requested to do so, therefore for 99.9 recurring percent of the time there need be nothing observable.

[alexwright]

Or reply in some side channel, piggy back the next (expected, ie when the user has switch back to normal mode) UMTS radio packet for example. I don't know the packet structure but I expect there are areas that could be re-purposed covertly. We did after all fit the entire SMS system into such a space.