Two-way communication between a general purpose computing device, such as a PC, and a mobile terminal, such as a cell phone. The general device executes a conversation user interface through which a user enters a message. The message is sent to an intermediary communication service, such as a portal server, along with a mobile terminal identifier, such as a phone number. The intermediary service creates a mapping between the mobile identifier, a general device user identifier, and an extension to an intermediary device identifier, such as an extension to an SMS short code. The intermediary service sends the message to the mobile terminal along with the intermediary service identifier and the extension. The mobile device returns a response with the mobile identifier, the intermediary service identifier, and the extension. The intermediary service uses the mobile identifier and the extension to determine the general device user identifier and send the response.