Disclosed is a communication system for effecting communication events between a plurality of endpoints connected via a communication network. The communication system comprises a plurality of processing units other than said endpoints, each having access to computer storage holding executable code modules for managing a communication event. The code modules are configured to implement a media modality controller configured to manage media modality of an established communication event and a call controller configured to establish the communication event. An instance of the call controller is assigned to progress the establishment of the communication event responsive to an instruction received via the network and is configured to initiate an instruction to at least one of: the media modality controller; and at least one of the endpoints.