A real-time communications system that is capable of convening an on-line meeting or teleconference involving text, audio, video, and/or data between multiple users of mobile and non-mobile devices. A mobile telephone is operative to receive an invitation to join an on-line meeting with at least one user of another mobile or non-mobile device. If the mobile telephone network can provide an indication of whether the mobile telephone is powered-on and within the coverage area of the mobile telephone network, a real-time messaging server employs this information to determine the on-line presence of the mobile telephone user before transmitting the meeting invitation. Otherwise, the real-time messaging server assumes that the mobile telephone user is present and on-line. The mobile telephone user may indicate his or her desire to join the on-line meeting by transmitting an SMS message to the real-time messaging server in response to the meeting invitation, after which the real-time messaging system may convene the on-line meeting between the multiple users.