I did a snopes search on this one and it can neither be confirmed or denied. It is side-splitting:

An operations manager for Jack in the Box was late for a meeting and called his boss to let him know he was running late. As the manager was leaving the voicemail message, he witnessed an accident and went on to provide a "play by play" account of the incident. This is the actual voicemail message. It was forwarded so many times within Jack in the Box that it crashed their voicemail server.

Jack in the Box Voice Mail