'idle' = '0' = on-hook, idle state 'busy' = '1' = off-hook, busy state 'forcedIdle' = SP = both A and B signaling bits are forced to '0' during out-of-service periods. 'forcedBusy' = MK = both A and B signaling bits are forced to '1' during out-of-service periods. 'busyIdle' = MK_SP = A and B bits are forced to '1' for 2.5 seconds; then they are switched to '0' until out-of-service condition disappears. 'idleBusy' = SP_MK = A and B bits are forced to '0' for 2.5 seconds; then they are switched to '1' until out-of-service condition disappears. - for T1: '0'=0 ; '1'=1 - for E1: '0'=1 ; '1'=0