In order to locate and clear the Secure Temporary File you will need to install the Outlook Editor tool. You can find it and download it on this website:
Once you have installed the program, locate and run it from the Start menu. When you run it, you will see the option to empty the secure temporary file. Choose this option and let it run. Once completed, if Outlook is open, close it and reopen it.
Clearing this file will usually solve problems associated with opening voicemails in Outlook.