Along with the 500 features originally announced for Microsoft’s Windows Phone 7 Mango OS, Microsoft today announced the ability to create custom ring tones. Obviously, pre-loaded ring tones will still be available for those of us who enjoy a more classic ringer… but if you want Fergie to assure you that tonight’s going to be a good night every time your special lady friend calls, now you can.

The file must be less than 39 seconds long, smaller than 1MB, and be DRM-free MP3s/WMAs. Outside of those specifications, you basically have free reign over what you’ll hear when you get a call. You can set up ring tones with the desktop Zune client, by finding the file in Zune, right-clicking, and changing the Genre to ring tone.

Along with the ability to add custom rings, Microsoft is also hooking Mango up with 9 new on-board ring tones. Microsoft mentioned in the announcement that the new Mango SDK gave developers a number of cool tools to create ring-tone creation apps, so be sure to check for those once Mango arrives in Fall.

[via Windows Team Blog]