You can disable Message Monitor system wide using the UTIM (Telephony Integration Manager) in the Tools Depot. Expand your integration and go to Properties. Choose the Phone View tab and uncheck "Message Monitor Enabled". The only way I'm aware of to disable this on a per phone basis is to disassociate the phone with the user account used for the Phone View integration. This would, however, disable all Phone View functionality - Message Monitor, Visual Message Locator and Remote Message Monitor. I'm not aware of any way to change the volume of the alert.