This is a read-only counter, in milliseconds, tracking how much voice playout was synthesized from parameters or samples both preceding and following the gap in time, due to voice data arriving late or being lost. An admin uses this as another gap-fill category alongside the silence and prediction counters, together painting a fuller picture of how packet loss is being concealed for this call. For example, comparing the relative sizes of the interpolation, prediction, and silence gap-fill counters could tell an admin which loss-concealment strategy is doing most of the work masking network problems on this call.