Phase noise can significantly degrade the performance of a block-coded digital communication system. This degradation can be mitigated by interleaving. The bit error rate (BER) of a block-coded system is estimated, taking into account the correlatedness of phase noise, as a function of interleaver depth. In principle, simulation can rigorously solve this problem, but the computer run time is prohibitive. A simplified system model that leads to an analytical formulation amenable to computer solution is presented. Numerical difficulties arise when a standard approach is used for long block codes with relatively high error-correcting capability and moderately small channel BER. An alternative formulation that is numerically much more stable is developed.>