Mitel 3300 exchange 2010 integration

[headbite]

We are running our voicemail using exchange. There is an option to "Play on Phone". This is hit or miss. When you click the option you get a dialog box to input your extension. Afte you enter your extension and hit the dial button the phone rings but sometimes you get audio and sometimes you don't. Has anyone out there seen this. Is there something I should look at in a wireshark trace?

Thanks in advance
Mark