The call detail data is temporarily stored in a call detail data pool on the SWU so that there is no loss of call detail data if there is an ISp outage or outage of an output device. The size of the memory depends on the system type. The ECX is designed for the temporary storage of a maximum of 3800 CDR records. Every other type of system is designed for the temporary storage of a maximum of 2200 CDR records. When the limits of this memory capacity are reached, no more call detail data can be temporarily stored and call detail data is lost. Therefore, to minimize the loss of call detail data for outgoing trunk calls (expensive connections), the internal connections are no longer recorded once the call detail data pool has been filled to 75% of its capacity. Before this point is reached, appropriate error messages (alarm messages) are sent.