Reports the number of users connected via the callback service, where the device dials the user back rather than accepting the inbound call directly, on the access board in this chassis slot (the table is indexed by slot number). An admin polls this counter alongside its sibling userStatisticsBoard* objects to see how the mix of dial-up service types is distributed for this particular board (this table's own source DESCRIPTION notes the callback service is not supported on release R100); it lets an admin see how load is distributed across individual boards, e.g. to spot one card carrying a disproportionate share of a given session type.