If your signup flow sends a text message, you already know it costs money per send. What's less visible on the invoice is everything SMS OTP costs you that never shows up as a line item: the users who bounce waiting for a code, the toll-fraud traffic nobody noticed until the bill doubled, and the engineering hours spent handling carrier edge cases that have nothing to do with your product.
This isn't an argument that SMS is broken in some abstract sense - it's an argument that for an auth step specifically, you're paying for properties you don't need and not getting the one you do.
The direct cost is the easy part to see
Twilio-style SMS pricing for OTP sits roughly in the $0.03–$0.08 range per message depending on destination country, and that's before you account for delivery retries when a carrier silently drops the first attempt. At 10,000 monthly active users, each going through login at least once, you're looking at somewhere between $300 and $800 a month - for a step that, conceptually, is just "prove you control this identifier."
Compare that to what a messenger-based login costs structurally: nothing per-message, because there's no message to send through a paid carrier route. Bondify's Pro plan covers 10,000 MAU at a flat $20/month, with overage at $0.02 per additional MAU beyond that, paid from a balance you top up once - no auto-charging.
