In this paper, the authors study the physical-layer identification of GSM devices. For their exploration, they build an ad-hoc acquisition setup that collects GSM signals during voice calls. They collect signals from a population of 18 mobile devices and build fingerprints by considering both the transient and the data parts of the acquired signals. Their results show that devices of different models and manufacturers can be identified with high accuracy (0% identification error) by exploiting transient-based fingerprints. Same model and manufacturer devices could also be identified by using transient-based fingerprints: they find an identification error between 0 and 8% depending on the considered device set.