Have you tried calling an apprentice and never get a response?
Sometimes this can happen because the apprentices may get so many phone calls that they cannot possibly get back to everyone, but wait! That does not mean that you still can't get a FREE service.
Sometimes people that call or leave an email message do not have the type of hair that the apprentice needs for a specific class, in fact many times some models do not even show up when they are booked. It takes a long time to filter through all the models to find the volunteers that will work - then there is finding the schedule that works for both the apprentice and the volunteer. Remember, most apprentices are doing this using their own email accounts and cell phones.
If you want to increase your chances of getting a FREE service, request to be a "standby model" in case someone cancels. You can also try calling the salon directly to find out if any models are needed because of cancellations.
If a volunteer does not show up, then the apprentices, most likely, are near their cell phones so you can call for an immediate appointment.