A condition of an interface, as determined by the network, based on the network's capability to terminate another call to that interface. The interface is said to be NDUB if any of the following conditions exist: (a) if the maximum number of information channels available has been reached and the network does not support the offering of additional calls beyond this number; (b) if the limit of additional offering had been reached; (c) if the maximum number of total calls supported for User B has been reached.