With mine, it goes through to a call centre out of hours. They take details and phone number and the vet then rings me back. I guess it will vary from practice to practice. I would assume with an answer phone, the vet will pick up the messages and get back to you. Only problem with that is if the vet is with another pet, s/he will not be able to ring you back immediately. I doubt there will be more than one vet on call.