AFAIK, I don't think there's a solution. Looks like you're gonna have to contact the company and ask them to send you the source code for their program, and try compiling it on Linux! :P Hehe... Sorry, that wasn't very nice.
Nope, it's their own application, and unless there's a way of accessing it with another VOIP app through _that_ protocol (not mentioned) then I'm afraid you won't be able to use it.