What causes this: NewCrop has found that some carriers will abruptly start blocking a SMS message (a type of text message used for the OTP).
The fix: To verify that this is the case, text START to 25370. You should receive an immediate response letting you know that you can receive messages. If this does not work, contact your phone service carrier to have this unblocked. This can take a few days so the provider must use another method to receive the OTP in the meantime.