What is the reasoning to using an IP system in the first place? The STS (for the most part) is a rock solid product for a business application that is not in need of IP features. If you are looking for a true replacement for a digital key system, I believe the Asterisk will fall short for you. However, you could make it work. For deploying it alongside the STS I have not herd of that, but others here can probobly help you out more.

Steve