While setting up an asterisk pbx, I found out that Sprint has really shitty authentication for accessing thier customers voicemail boxes. Basically, if you can spoof your caller ID number to be the number of the voicemail box you want to access, it will dump you right into the users new messages. This will only work if they don't have thier box protected by thier pin which is an option Sprint customers can turn on, but I don't think it's a default.