Methods and computer program products for establishing communication events between an initiator user and a recipient user over a first communication system. The initiator and recipient users are contacts within a second communication system but are not contacts within the first communication system. For the initiator and recipient users, an association between a first user identity in the first communication system and a second user identity in the second communication system is determined. The initiator user device uses the association for the recipient user to determine the first user identity of the recipient user. A setup request to establish the communication event is sent over the first communication system from a first initiator client to a first recipient client. A communication event is authorized to proceed over the first communication system on the basis that the initiator user and the recipient user are contacts within the second communication system.