Disclosed is a communication server that includes a message transfer unit that transfers messages sent between a first user communicating under a first communication system and a second user communicating under a second communication system. For example, the communication systems could each comprise a plurality of mobile wireless transceivers and a plurality of land-based transceivers that are used by emergency-response organizations. These communication systems may be incapable of communicating directly with each other. The communication server also includes a translator connected to the message transfer unit. The translator translates messages sent from the first communication system into a format compatible with the second communication system and vice versa. The communication server also has a voice-over-Internet-protocol (VoIP) unit connected to the message transfer unit, so that messages are transmitted through the communication server in a VoIP format. Thus, messages are transferred between the communication systems using discrete Internet protocol addresses.