If an attempt by a first user to contact another party fails, a callback server sets a flag. The callback server monitors terminals that have such flags set and is arranged to transmit an alert to the calling party when the called party next becomes available. However, if at the time when the called party becomes available, the calling party is himself no longer available, being now switched off or busy, an additional flag in the store, indicative of activity by the calling party, prevents a callback alert from being generated until the monitoring process can obtain availability indications in respect of both parties simultaneously. Availability may be determined by factors such as available bandwidth or terminal capabilities. A callback may be initiated when a predetermined number of called parties become available to participate in a conference call.