A digital network based communications system that establishes communications sessions to communicate via a simulated circuit-switch environment. Exemplary embodiments communicate between a first communications station (104) and a second communications station (106) using Voice over Internet Protocol (VOIP) techniques. Calls are setup (700), maintained and torn down (800) using Session Initiation Protocol and Session Description Protocol messages. Call modes are able to be changed between duplex and simplex without disconnection. Duplex calls are also able to be interrupted with a request for a simplex call. A receiver (104) engaged in a duplex call can receive a request for a simplex call, accept the simplex call or send a response message, and then resume the duplex call, all while maintaining the duplex call data communications session.