A method of establishing communication between a first and second terminal, the first terminal being arranged to communicate with a first communication system and the second terminal being arranged to communicate with a second communication system. The method comprises the steps of: a user of the second terminal selecting an identity to represent the second terminal in the first communication system and registering a network address of the second terminal at a first storage means; allocating to the second terminal a calling identifier from a set of available calling identifiers and mapping the network address to the calling identifier at the first storage means and mapping the identity to the calling identifier at a second storage means; a user of the first terminal initiating communication with the second terminal using the identity; the first terminal translating the identity to the calling identifier using the second storage means and transmitting a message to the first communication system to initiate communication with the second terminal using the calling identifier; responsive to receiving the message at a network node of the first communication system, the network node translating the calling identifier to the network address using the first storage means; and establishing a connection between the first terminal and the network node over the first communication system, and between the network node and the second terminal over the second communication system.