In this case, lossless is not to be expected. Since Lightning-to-Lightning cables do not seem to be available at the moment, I assume that it is an older iPhone with a 3.5 mm audio connector. In this case, the signal from an analogue source (3.5 mm connector) is converted to a digital signal by an analogue-to-digital converter. With digital conversion in the cable, the playback is not completely lossless, but still very high quality.