Voicemail notification sound

[G671]

I can't seem to change my Voicemail alert sound, using custom audio files. From the main settings/sounds/notifications, I've set my alert sound for "notifications". And, by default, the phone uses that sound for voicemails, too. But, when I open Voicemail, and go into Voicemail settings to change it from there (to a different custom sound), it won't let me change it. My custom sound files show up, but it won't take the change. It will, however, let me change it to a stock sound file (ogg).

Any tips? I want a different sound for Voicemail, the one I'm using for general notifications.