There is a new tool currently in beta just out from Microsoft. The Exchange UM Troubleshooting Tool.
The Exchange 2010 UM Troubleshooting Tool is an Exchange Management Shell cmdlet that you can use to diagnose configuration errors specific to call answering scenarios and to test whether voice mail is functioning correctly in both on-premises and cross-premises UM deployments. You can use this cmdlet in deployments with Microsoft Office Communications Server 2007 R2 or Microsoft Communications Server "14" or in UM deployments with IP gateways or IP PBXs.
This cmdlet emulates calls and runs a series of diagnostic tests that help on-premises administrators to identify misconfigurations in telephony equipment, Exchange Server 2010 SP1 Unified Messaging settings, and connectivity issues between on-premises and cross-premises deployment of Exchange 2010 SP1 Unified Messaging.
When you run the cmdlet, it states the reason and possible solutions for issues that have been detected. It also outputs general audio quality metrics for diagnosing audio quality issues related to network connectivity such as jitter and average packet loss.