It's not a derivative of the presentation of the artistic elements of a work. It's derived from the technical working of a piece of software.

The elements of the software doing novel technical stuff might be patented, but copying the technical working is allowed. Copyright doesn't protect technical aspects.

If the project copies the UK of Skype, then that's a tort. If it copies the written code or uses the human reading of that code to derive a new codebase, then that's a tort. If it copies the technical implementation, how memory is accessed, what codecs are used, what packets are sent and when ... that's not copyright infringement.