I don't use the Ubuntu, but assuming everything else being the same, what do you have for your IVR for Timeout? Set it to 1. I have a voicemail IVR with a backdoor in it. We play the message to leave a message - set timeout to 1, invalid retires disabled, timeout retries to 0 and timeout destination to voicemail. If someone doesn't press the magic code within 1 second of playing the voicemail recording, they hear a beep and leave a message.