A two-party line split between "tip party" and "ring party" could be created in such a way as to allow the central office to determine which party placed an outbound toll call by detecting that one of the ringers was disconnected when that subscriber went off-hook. This system would fail if any provision was made to allow the subscriber to turn off the bells (do not disturb) for privacy or unplug the telephone; it also presumed that each subscriber only had one telephone connected to the line.