The B channel number for this active entry. BRI interfaces have B channels in the range 1..2, PRI interfaces
have B channels in the range 1..31, excluding a channel which is the D channel. A value of 0 for this entry means
that either a B channel is not currently allocated for this call, or that more than one B channel is allocated.
In the second case, the management client will have to walk the B channel allocation table looking for channels
with the same active call index as this call's.