Text messages go out with the first reminder through your own Twilio or Vonage account, or as an approved template through the WhatsApp Cloud API. They go only to customers who tick a separate checkbox in the classic checkout or the Checkout block and have a phone number on the order. Numbers without a country code use the billing country. A failed message leaves a note on the order, and the email still counts.
For WhatsApp, create a template in Meta with three body values in this order: first name, products, reorder link. Keep the {stop} placeholder in your SMS text.
Your account, your rules. Sender registration, template approval and local law are between you and your provider. These connections were tested against mocked replies, so send yourself a real message before going live.