Can you use an MP3 as a ringback tone? If so, how do you set it up on a Motorola Droid?
http://mediastore.verizonwireless.com/onlineContentStore/index.html# For Ringback Tones.
It is my understanding that Ringback tones are not specific to your phone but rather coded to your account. With that said I wouldn't believe you can upload custom ringback tones due to legality issues for VZW
The information crb79 provided is correct! Here is a link for additional information on ringback tones http://support.vzw.com/clc/features/data_services/ringback_tones.html (scroll down to Are Ringback Tones downloaded to my device)