Slydial is a US-only service which allows you to leave voice mail messages for some-one without ringing their phone at all. As the name implies, this is obviously good for getting oneselves out of a sticky situation, but as the company suggests, its also useful for conveying more information than a text message would allow while while still taking much less time than a conversation would normally occupy.
Slydial has now made a native app for a number of platforms, including Windows Moble. The application can work in two ways: Firstly as stand-alone app and secondly integrated into the contacts app, which allows dialing Slydial directly as a menu option when you search your contacts.
You need to register to download the application from Slydial here.