A method of establishing a communication event between an initiator user and a recipient user over a first communication system which requires users to have a contact relationship to communicate with one another, and wherein a first initiator client implemented at an initiator user device enables the initiator user to access the first communication system, and a first recipient client implemented at a recipient user device enables the recipient user to access the first communication system, and wherein for each of the initiator user and the recipient user, an association between a first user identity in the first communication system and a second user identity in the second communication system is determined, characterised in that: wherein said initiator user and said recipient user are contacts of one another within a second communication system but are not contacts of one another within the first communication system, the method comprising: at the initiator user device, accessing contact details within the second communication system to determine the second user identity of the recipient user, and using the determined second user identity of the recipient user and said association for the recipient user to determine the first user identity of the recipient user; proving the second user identity of the initiator user within the first communication system to the recipient user by exchanging certificates between the initiator user and the recipient user, wherein the certificates being issued by a server in the first communication system; receiving the second user identity of the recipient user in the first communication system from the recipient user by exchanging said certificates between the initiator user and the recipient user; sending a setup request to establish the communication event over the first communication system from the first initiator client to the first recipient client, said setup request being sent using the determined first identity of the recipient user, wherein information is provided to the first recipient client indicating that the first identity of the initiator user is associated with the second identity of the initiator user, and wherein the communication event is authorized to proceed over the first communication system between the first initiator client and the first recipient client on the basis that the initiator user and the recipient user are contacts of one another within the second communication system. at the initiator user device, accessing contact details within the second communication system to determine the second user identity of the recipient user, and using the determined second user identity of the recipient user and said association for the recipient user to determine the first user identity of the recipient user; proving the second user identity of the initiator user within the first communication system to the recipient user by exchanging certificates between the initiator user and the recipient user, wherein the certificates being issued by a server in the first communication system; receiving the second user identity of the recipient user in the first communication system from the recipient user by exchanging said certificates between the initiator user and the recipient user; sending a setup request to establish the communication event over the first communication system from the first initiator client to the first recipient client, said setup request being sent using the determined first identity of the recipient user, wherein information is provided to the first recipient client indicating that the first identity of the initiator user is associated with the second identity of the initiator user, and wherein the communication event is authorized to proceed over the first communication system between the first initiator client and the first recipient client on the basis that the initiator user and the recipient user are contacts of one another within the second communication system.