An "Intelligent" telecommunications network comprises a plurality of switching units interconnected by links and connected to a central computer unit by a data communication system. For dynamic routing of a call, a first switching unit responds to a destination address in a call to attempt a direct link to a neighboring switching unit and, in the event that the attempt is unsuccessful, issues to the central computer unit a message containing the destination address. The central computer unit uses the destination address to identify the unsuccessful link; (ii) updates a routing database to identify the link as unavailable; (iii) determines an alternative route for the call using a tandem node and (iv) compiles a return message including a network address for a tandem switching unit and transmits it to the first switching unit. The latter attempts to route the call via a link to the tandem unit which attempts to complete the call by a direct link to the destination switching unit. The tandem node switching unit also queries the central computer if it cannot complete the call by a direct link to the destination switching unit, whereupon the central computer will set to zero the idleness of the direct link which the tandem unit attempted and then determine an alternate route from the tandem unit to the destination, i.e. with the idleness factors of both direct links set to zero, resulting in an alternate route having three links.