There are various legal bases which may authorise you to send SMS.
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It is advisable to obtain the consent of your recipients before sending SMS messages. To be on the safe side, we also recommend that you use a double opt-in. The first message you send to your new recipients should therefore be a request for confirmation of consent. If the recipient does not agree again, you should not send any further SMS to this recipient.
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You can read more details about the legal basis in our blog post.