Although the behavior of subducted slabs in the mantle transition zone remains uncertain, images from seismic tomography provide insight into a rather complex geodynamic evolution of slabs in this region. Using a numerical mantle convection model we consider in more detail the dynamics of subducted slabs around an endothermic phase change at 660 km depth. The time-dependent temperature fields from the flow simulation are used to derive perturbations from a radially stratified seismic velocity model. We find that the positive velocity anomalies of the cold descending slabs may be significantly decreased at the phase change owing to the slab-induced downward phase boundary deflection and associated velocity discontinuity across the boundary. The slab may be completely or partially rendered seismically undetectable, depending on the amount of phase boundary deflection. The results have implications for the seismic interpretation of the dynamics of subducted slabs interacting with a phase transition.