Almost 20 years ago in the UK I used a unit designed for British rail unmanned stations which you could dial into and leave a message which would then be played back according to a pre-programmed schedule. I used it to get around a feedback issue but it should still be available and may suit your needs. Unfortnately having no furher need of the information for the past 20years I can't remember the name of the product but hopefully this will give you a starting point in your search!