In Pb1-xSnxTe, a band inversion is known to occur as a function of composition, and leads to a topological crystalline insulating phase (TCI). This inversion is also referred to as the topological phase transition. When the Sn content x is chosen to be close to the critical inversion point (x=0.40), the band inversion from topological to trivial can be driven by increasing temperature. In this presentation, I will begin by introducing the idea of topological ‘protection’ by crystalline symmetry that leads to emergence of massless Dirac surface states in Pb1-xSnxTe. I will then present results on Shubnikov-de-Haas oscillations in Pb1-xSnxTe for x=0.46 using pulsed magnetic fields. This compound is a TCI at 4.5K, but goes through a topological phase transition at T≈70±10K, and becomes a trivial (non-topological) semiconductor. Tracking the Shubnikov-de-Haas oscillations up to 60T, allows to observe Landau levels that survive up to 100K, and to quantify the shape of the Fermi surface as well as the Berry phase of the system as it goes through the band inversion. We do not see any change in the area of the Fermi surface between 4.5K and 100K, we do, however, observe a continuous and dramatic shift in the phase of the oscillations. This suggests that the Berry phase in Pb1-xSnxTe exhibits a smooth variation, approaching zero as the system goes through the topological phase transition, and ceases to be topological. Since the Berry phase is a true measure of the massless Dirac nature of surface states, our observations provide insight into the mechanism of mass acquisition in condensed matter systems.