It's the original voice but it will only work if the driver inputs the code correctly. Only a few of the old hands know the codes.
The HST buffet announcements will play throughout the whole train but only the 'big buffet' vehicles have the Gaitronics keypad handset that allows it. There are four announcements, three are 'welcome aboard' announcements, one for peak time service, one for weekday daytime off peak service, and one for evenings and weekends. They reference the Refresca Cafe Bar and old coach letters so aren't really much use other than as a handy PA test if you're prepping the train.
The fourth one is a termination announcement: "Ladies and Gentlemen, thank you for travelling with First Great Western. Please make sure you have all your personal belongings with you and please mind the gap when stepping from the train to the platform. Your assistance in closing the train doors behind you will be appreciated."
I like using that one in termination situations, it's handy for waking the dead