We study quasinormal modes of black holes, with a focus on resonant (or quasinormal mode) expansions, in a geometric frame based on the use of conformal compactifications together with hyperboloidal foliations of spacetime. Specifically, this work extends the previous study of Schwarzschild in this geometric approach to spherically symmetric asymptotically flat black-hole spacetimes, in particular Reissner-Nordström. The discussion involves, first, the nontrivial technical developments needed to address the choice of appropriate hyperboloidal slices in the extended setting as well as the generalization of the algorithm determining the coefficients in the expansion of the solution in terms of the quasinormal modes. In a second stage, we discuss how the adopted framework provides a geometric insight into the origin of regularization factors needed in Leaver’s Cauchy-based foliations, as well as into the discussion of quasinormal modes in the extremal black-hole limit.