In a method of routing calls to an external network from a calling node of a private network having a plurality of external network access circuit groups, each circuit group is connected to a gateway node of the private network. The method calculates a cost for different possible routes to the external network, the cost of a route to the external network being a function of the circuit group used and the route in the private network between the calling node and the gateway node of the circuit group used. It also chooses a route as a function of the calculated costs. The cost of a route to the external network is a decreasing function of the resources available on the circuit group used and is also a decreasing function of the load of the links used for routing in the private network. This optimizes use of the resources of the private network.