In Client Config > Business: Website, add Optin SMS to Fields to ask on Register (it is included by default) — the registration form then shows the consent checkbox "I agree to receive SMS messages from <Company> for alerts, order updates, and notifications. Message and data rates may apply. Message frequency may vary. Reply STOP to opt out or HELP for help." with links to your privacy policy and terms. To force customers to tick it, also add Optin SMS to Fields mandatory on Register. The checkbox is unchecked by default, which is what the A2P campaign registration describes as your opt-in flow, so keep the wording and the unchecked default.