Processing signalling relating to a telephony session in a first telephony network. The session involves a telephony device associated with a second telephony network. A first message received from a first node in the first network comprises a first telephony identifier, associated with the first network, as a called party identifier. A second, different telephony identifier, associated with the first network, is identified. A second message transmitted to the first node comprises the second identifier as a telephony party identifier, whereby to enable call services to be provided in the first network on the basis of the second identifier. A third message, comprising the second identifier as a telephony party identifier, is received from the first node. A third telephony identifier, associated with the second network, is identified using the second identifier. A fourth message, comprising the third identifier as a telephony party identifier, is transmitted to the first node.