This Counter64 counts the number of times this pseudowire's playout (PDV/jitter) buffer ran completely empty because expected packets had not yet arrived. Because this is the cumulative TdmPwTotalStatistics counter for the pseudowire, tallied since the bin's own start time rather than being reset every short collection interval, each underrun forces the device to substitute a fill pattern for the missing TDM payload, which is heard or measured as a defect on the emulated circuit — this is one of the two counters (with Overrun) that most directly indicates the jitter buffer is undersized for the packet network's actual delay variation. For example, a rising Underrun count that coincides with a path that has high packet-delay variation would justify increasing this pseudowire's configured jitter-buffer depth rather than assuming the TDM hardware itself is at fault.