I receive sms messages from both a taxi service(on way/arrived) I use and a physiotherapy clinic(reminder that you have an appointment tomorrow at ...) that I use. I find these useful and am grateful to receive them. On the other hand if they were automated voice calls I would hate them with a passion, I just can't describe how much I hate being called by an automatic service. SMS seems like the right way to go. Given that you do that I think you have a valuable service that I would be happy to use.