For me VOX sensitivity of the 10U is too low. I need to shout really loud to get it activated (e.g. to accept an incoming phone call).
I have the same problem with 2 owned deviced and I tested a brand new device of my retailer with the same problem, so I assume that the firmware needs to be adapted.
I tested with 1.0, 1.0.2, 1.1, 1.2 and 1.2.1 all have the same problem. With 1.0 it seemed to be worse that with the others.
Some ppl. have the problem in the opposite direction (VOX reacts on wind noise). Seems like this problem is specific for the Schuberth C3 Version where sensitivity is too low and for others it is to high.