A presence trigger system and a service system support automatic actions on a voice conference based on the presence state of potential conferees of the voice conference. The systems may, for example, initiate contacting and joining the potential conferee to the voice conference when the potential conferee presence state indicates that the potential conferee is available. As another example, the systems may automatically notify participating conferees that the potential conferee has become available. Automatic notification allows participating conferees to concentrate on and contribute to the voice conference without the distraction of frequently checking the presence status of expected, but absent, conferees.