Initiating a conference call includes receiving a conference call request message from an initiating user specifying a plurality of other users and determining an availability of the other users to receive conference call invitations. A conference call is initiated between the initiating user and one or more of the other users that are determined to be available. The availability of one or more of the other users determined to be unavailable is monitored and interaction with a conference call invitation by one or more of the other users determined to be unavailable is enabled upon their monitored availability changing from unavailable to available. The interaction with the conference call invitation by the other users is conditioned on whether the conference call is still in progress.