If you're trying to test out scheduling reminder SMS messages, and receive an error, the most likely cause is that you've opted out of another SMS service from Twilio, our provider. Luckily it's an easy fix:
Text UNSTOP to +14153902261
Twilio powers everything from Uber to political campaign texts, and usually opt outs via SMS remain per service, but sometimes they can honor your opt out across their entire network, even if you've never received a text from Ethnio.
This is fairly rare but a small percentage of your participants could also not receive SMS reminders for the same reason. In our experience it's <2% of participants.