please make sure you're using your full phone number incl. country and area code formatted like this:
with no dashes or spaces. Then try to use the "send a test" option in Preferences -> Reminders and repeat this test selecting each of our 3 outgoing hosts (from the "Number to send from" drop-down menu).
If neither the US, nor the Canadian hosts work, please send your full phone number to firstname.lastname@example.org and we will contact our SMS provider about this issue.