The prediction of fracture is of utmost importance regarding the design of modern, specifically tailored engineering materials. These materials are often heterogeneous, i.e. their properties vary in space. This can either be achieved purposefully by combining two or more constituents to profit from a more resilient composite material, or happen due to unavoidable imperfections. In any case, purely experimental assessment of failure is tedious and circuitous as soon as the structure of interest gets more complex, and the involvement of numerical models is inevitable. The phase-field approach to fracture is a powerful methodology which is able to capture manifold crack phenomena inherently and sidesteps the need for remeshing by describing the crack as a continuous field. In this contribution, the phase-field model is extended to a fully diffuse incorporation of heterogeneities: A static order parameter smoothly switches between two bulk material constituents, while the weak, brittle interface is incorporated via a continuous fracture toughness distribution. The sharp interface jump conditions still hold for the diffuse representation since a partial rank-1 relaxation is employed in accordance with the unilateral contact condition of the phase-field model. Moreover, a compensation procedure ensures the independence of the interface fracture toughness from interface and crack phase-field length scales. The model is validated by a comparison to analytical results and the predictive power is demonstrated by the deduction of direction-dependent, effective fracture properties of heterogeneous microstructures.