The source texts of the D-Modem project, which implements a software modem for organizing data transfer over VoIP networks based on the SIP protocol, have been published. D-Modem makes it possible to create a communication channel over VoIP, similar to how traditional dialup modems allowed data to be transmitted over telephone networks. The areas of application of the project include connecting to existing dialup networks without using a telephone network at the second end, organizing covert communication channels and testing the security of systems accessible only through dialup. The project code is written in C language and distributed under the GPLv2 license.