Techniques are disclosed for determining, for each candidate communication medium, the availability of one or more telecommunications endpoint users for participating in a communication session of interest (i.e., either new or existing) via the evaluated medium. In accordance with the illustrative embodiment of the present invention, an availability manager determines the availability based on (i) one or more discrete rules, or (ii) one or more continuous functions, or both. The continuous functions are specified with respect to the time that has elapsed since a previous session, the time remaining in an active session, and the expected duration of a new session. Once it has determined the availability of one or more participants in the communication session, the availability manager of the illustrative embodiment transmits one or more signals to a call-processing server, which then sets up the participants on the session via the appropriate communication medium such as voice, instant messaging, and so forth.