A common challenge when deploying Lync in an enterprise voice environment, is to have the Lync Client behave as close to an “ordinary” phone as possible.
One common “problem” is the busy-on-busy. When an incomming call is routed to a Lync Client already in a call, you would want the caller to get a busy tone. This is, by default, not a function in Lync. To get this behavior, one would have to do it by using MSPL scripting as referred to in this article(not tested, so I don’t know if this actually Works) or use a Third party Application.
I’ve recently deployed this in a small environment using Busy-On-Busy from UnifySquare.
This is a fairly cheap solution, and it’s very easy to set up and configure. The functionality is also delivered in larger call center applications/suites like Competella Unified Communication Suite for Microsoft Lync.
You decide which is the best option in your environment :)