May I send SMS marketing to my customers?

SMS = electronic direct marketing — falls under Dutch Telecoms Act Art. 11.7 + GDPR. Stricter than email in public perception. Rules: (1) Opt-in: explicit consent required before first SMS. NOT "tick a box at checkout" — if box defaults checked = no valid consent. (2) Existing customer relationship: exception (Tw 11.7(4)) — on existing customer contact + similar service + opt-out in every expression. Similar = not "we now have insurance" if you bought electricity from us. (3) STOP keyword: every SMS must have "STOP" as unsubscribe option — free, 1 SMS = unsubscribed. (4) Short ID: SMS must clearly identify business. No anonymous numbers. (5) Time rules: not before 09:00 or after 20:00, not on Sun/holidays for consumer marketing. Fine risk: ACM fines 2023-2025 up to €900k per violation. Common mistakes: purchase confirmations with "we'll also send you deals" without separate consent, won-prize SMS to database members, late STOP confirm (must be within 24h). For customers: STOP-reply must work. Doesn't work? ACM report + AP complaint.