We propose a measure of the reliability of a telecommunication network based on the expected value of the lost call traffic due to failures in the network. The evaluation of the lost call traffic in the failed network will be performed by resolving re-routing problems. The first formulation to be presented for the re-routing problem is a multicommodity model with capacity constraints. Algorithms to solve this kind of problem are extremely time consuming. We then propose a new formulation for which the problem is modeled as a multicommodity one with nonlinear costs and without capacity constraints. The objective estimating the lost call traffic has been chosen to take the stochastic character of the demand into account. This new model solves the problem more easily and represents an approach closer to reality since stochastic demand is considered. An existing convex-simplex algorithm is suggested to solve this problem efficiently.