If you're a developer using Unified Communications Managed API (UCMA) 3.0 with Lync Server 2010, you may need to add your workstation to a pool of trusted application endpoints so you can debug from your developer environment.
Nicholas Fish, answers: What's a trusted app endpoint? Why do I need a trusted app pool? How do I provision all that? How do I verify it worked? How do I configure my app to use the trusted app endpoint in Visual Studio 2010?