Method for determining a free connecting line in a switching center having a number of subscriber access units

A switching center comprising a number of subscriber access units which are controlled by a switching center, wherein

• the subscriber access units and at least one line trunk group in the switching center are connected to one another via a switching network and/or internal lines, via which they can pass on calls between their connections,
• at least one of the subscriber access units is connected via a group of transmission lines, which comprises a number of transmission lines, to in each case one other switching center,
• at least one of the at least one group of transmission lines of a subscriber access unit and a group of lines which has a number of lines of the line trunk group form at least one logical group of lines, which connects the switching center to the other switching center,
• when an incoming or outgoing call which is intended for another switching center occurs in a switching unit, this call is passed on via one of the lines in the group of lines to the other switching center, and
• the switching center is set up such that when an incoming or outgoing call occurs at the subscriber access unit, a check is carried out to determine whether a connecting line which leads away from this subscriber access unit in the group of lines is free for a call to the desired switching center, and is furthermore set up such that if one of these connecting lines is free, the switching center sets up a connection to the other switching center via this connecting line.