Call Control entities in a network communicate between themselves by amending call processing messages to include encrypted network information. As such, a call may be established whose path through the network is dependent on the paths of other calls. Information of a scope larger than a Call Controller normally possesses can, as a result of this communication, be made available to Call Controllers for constraining call establishment. This information could relate to other calls and connections associated with those other calls. The information may also relate to gateways in and to adjacent networks and the Call Controllers in the adjacent networks that are related to the current Call Controller.