Ethnio allows you to automatically remind your participants of study times via email, text message or phone call. You can set these up in Scheduling >> Reminders.
Please note: You set the From / Reply-to from Scheduling >> Emails. And if you don't ask for a phone number, text reminders will not work.
Text messages come from the Ethnio (415) phone number, and if anyone replies, they get a nice message back asking them to reply to their scheduling email with any questions.