I guess it depends on whether you think you might stay on as a postgrad, if so I would definitely choose something in the area you think you intend to study.
Probably the most important thing is to get a supervisor for the final project that you can trust and will push you, in fact if you have a lecturer you think fits the role go ask him/her tomorrow (unless you're on hols) if they will consider being your supervisor
next year, and run your ideas past them - it helps if it is within their area of expertise. This'll also give you an idea of whether they are more interested in the implementation or in the research.
A VOIP project would be interesting although possibly a bit of a push given your other work, but don't forget the C# bit is irrelevant to the project, it is the VOIP that is of interest.