The Wiki of Unify contains information on clients and devices, communications systems and unified communications. - Unify GmbH & Co. KG is a Trademark Licensee of Siemens AG.
The open source software easy21653 is part of the OpenStage telephones in this list.
An overview of all used open source software on OpenStage telephones you find in the category page.
This contains amongst (many) other things the modified toolchains used for Openstage
The toolchains are derived from GPL source code.