First of all, if you have any kind of voice service handling your number such as Google Voice, Vonage, etc, or have an iPhone, this is a non issue because the number will show up on your visual voicemail (where it is probably also transcribed). But in the event you don't, most carriers provide "envelope information" that provide the number, date, time etc from the message. I know on AT&T you press 5 for this. Was this recorded on an old school tape message machine?