While Decipher VoiceMail is importing data, if it detects that backup encryption is turned on for a backup, you’ll be prompted to enter the backup password. This prompt means at some point backup encryption was turned on for this iPhone. Decipher VoiceMail cannot read the backed up voicemail data without the password.
You can skip that particular backup by pressing cancel, or you can try some passwords. If you don’t remember setting your backup encryption password, you may find this article about forgotten iPhone backup passwords helpful.