A system is disclosed for messaging between wireless mobile terminals operating on wireless carrier networks and networked computers. The mobile terminals and computers include client applications for communicating messages to one another using push-to-talk modality. A server, located on a packet network outside the wireless carrier networks, forwards messages between the mobile terminals and the computers. The messages consist of text or streaming voice. The server can also include gateways for forwarding messages from the mobile terminals and computers to external email and instant messaging (IM) users. By placing the server outside wireless carrier networks and using conventional packet network protocols such as the Internet protocol (IP), the system provides seamless inter-carrier push-to-talk and/or instant messaging between mobile terminal, networked computers, and users of third-party email and IM services.