Well, voicemail and extensions will work, but those trucks might have a little trouble traveling through 24 guage wire. Sorry, couldn't resist. Seemless, well, seemless is a very useless word. It all depends on what you are looking for, so far, you can do everything you asked. If your looking for coverage of another extension, it will take a little bit of programming. If your looking for busy indicators or primary coverage buttons over the UDP, forget it.