Mass, Inertia, and Gravity as Phase Anchoring: Why Matter Resists Change in a Phase Ontology (Phase Ontology — Part VI) develops a unified interpretation of “heaviness” by treating phase coherence as ontologically primary and spacetime as emergent. Instead of taking mass as an intrinsic substance-property (Newton), a purely geometric source term (GR), or an interaction label (QFT/Higgs), this paper proposes a single underlying mechanism: phase anchoring. In this framework, matter is a stable, long-lived phase configuration. Such configurations persist because they are anchored—coupled into surrounding phase relations in a way that prevents easy dispersal. Mass is redefined as the degree of anchoring: the deeper the anchoring, the harder the configuration is to modify. Inertia follows naturally: accelerating an object requires re-synchronizing and reconfiguring its anchored phase relations relative to the environment, which demands coherence redistribution. Gravity is reinterpreted as a coherence gradient produced by large-scale anchored structures: objects “fall” because phase relations reconfigure toward regions of greater coherence compatibility. The empirical equivalence of inertial and gravitational mass becomes expected, since both arise from the same anchoring strength. The manuscript also clarifies why mass appears localized (as a spatial projection of bounded coherence), relates the proposal to Newtonian mechanics, General Relativity, and Quantum Field Theory as compatible descriptive layers, and sketches speculative test directions (e.g., precision clock tests and experiments that modify coherence environments). Experimental falsifiability conditions are deferred to Part VII of the series.