Method and apparatus for routing telephone calls across different types of networks includes means for and steps of determining a destination IP address of a new call according to initial conditions established in a first network, determining a destination gateway (gateway brokering) based on the destination IP address, the initial conditions and a gateway selection policy, providing routing information of the destination gateway to the first network and completing the new call set up via messages received from the gateway brokering step. The initial conditions in the first network are established by determining if a callee is part of an IP network or part of a CS network and converting the callee phone number to an SIP URL if the callee is part of the IP network. Such determination is made by recognizing a prefix identifying an IP destination in the callee phone number.