Recently, cellular Ambient Backscattering has been proposed for cellular networks. Up to now an Ambient backscatter device, called zero-energy device or tag, broadcasted its message by backscattering ambient downlink waves from the closest Base Station (BS) according to a predefined pattern. A tag was detected by smartphones nearby. This paper presents, for the first time, a novel ambient backscatter communication system exploiting uplink ambient waves from smartphones instead of downlink waves. In this novel system, a BS connected to a smartphone monitors the uplink pilot signals and detects TAGs in proximity. The proposed system is implemented and tested with one prototype of TAG, a commercial off-the shelf 4G smartphone and a 4G Software Defined Radio (SDR) BS. Indoor and outdoor experiments were conducted to assess the proposed technique. These very preliminary experiments exhibit a promising potential. In indoor, a detection probability of more than 90% has been achieved without false alarm when the TAG was 3 meters from the UE, and the BS 20 meters away of them, behind walls and obstacles.