SMS phone verification (OTP) on a landing page form: is the friction worth it?

A quote-request form with type="tel" and format validation stops nothing: a randomly typed number passes any rule based on digit count or allowed characters, and no human will ever pick up when someone calls it. On an artisan landing page or an estimate page where a phone callback is the very next step, that kind of lead wastes a call for nothing — sometimes a follow-up too, a text message, a slot blocked in the sales rep's calendar. SMS code verification, or OTP ("one-time password"), solves one specific problem: proving the number entered is reachable at the moment of signup, not that it belongs to someone serious or reflects a real intent to buy.

Which forms OTP actually earns its place on

The extra step only pays off when the cost of a fake number clearly outweighs the cost of the added friction. Three form profiles meet that bar:

• Quote requests followed by a human callback — local artisan, mortgage broker, property estimate: every lead triggers an outbound call, and an invalid number is only discovered when someone dials it.
• Booking a limited slot — a free consultation, an on-site diagnostic, a supervised trial: a fake number blocks a slot someone else could have taken.
• A CPF or regulated form where lead fraud carries a direct cost, regulatory as much as commercial.

By contrast, a newsletter signup, a free lead magnet download, or a simple email contact form has no reason to demand an SMS code: the cost of a fake contact there is close to zero, while the friction stays full for 100% of legitimate visitors. The phone field can stay optional and unverified — that already covers what matters most for that kind of page.

What OTP proves, and what it doesn't

A code received by SMS and correctly re-entered proves exactly one thing: the number is active and its holder had access to it at that moment. It proves neither the person's identity, nor their creditworthiness, nor a real intent to buy — a visitor can verify a genuine number while having no intention of following through. That's an important difference from a disposable email address, far easier to generate in bulk: a verifiable mobile number carries a real cost to obtain, which mechanically limits fraud at scale. A study by Ballard, Cardwell and Young, published in 2019 in JMIR Public Health and Surveillance (available on Google Scholar), built and evaluated an eight-criterion fraud-detection protocol for online survey forms, and found that the validity of the phone number provided ranks among the most frequently triggered signals for flagging a fraudulent submission. The phone field, in this work carried out outside a commercial context but facing the same problem of mass fraudulent submissions, turns out to be one of the protocol's most reliable signals.

The friction cost: what the research on 2FA journeys shows

Adding a verification step is never free. Lyastani, Bugiel and Backes, in a study published in 2023 in the proceedings of the NDSS Symposium (available on Google Scholar), analyzed the two-factor authentication journeys of the web's top-visited sites and surveyed a panel of users about their experience: a notable share of respondents reported having already abandoned a site, stopped using it, or declined to activate a verification option because an authentication flow diverged from what they were used to on other sites. The mechanism they identified transfers directly to a form OTP: it isn't verification itself that drives people away, it's a flow that surprises them — an SMS that takes too long to arrive, a code field placed in the wrong spot, an entry error with no clear message. A well-designed OTP flow (code autofill, a visible countdown, an easy resend) keeps this loss small; a bolted-on one makes it worse.

Implementing SMS verification in Next.js

The implementation rests on three pieces: an SMS delivery service (Twilio Verify, Vonage, or the equivalent from your infrastructure provider), a server route that triggers the send and checks the entered code, and a client-side field optimized for entering that code.

// app/api/otp/send/route.ts (server-side, never exposed to the client)
export async function POST(req: Request) {
  const { phone } = await req.json();
  // validate the format (libphonenumber-js) before sending anything
  await verifyClient.verifications.create({ to: phone, channel: "sms" });
  return Response.json({ sent: true });
}

On the client, the autocomplete="one-time-code" attribute on the input field lets most mobile keyboards (iOS and Android) suggest the SMS code automatically, without the visitor having to leave the form to go read it in their messages — a simple friction win that needs no library at all. On Chrome for Android, the WebOTP API (navigator.credentials.get({ otp: { transport: ["sms"] } })) goes further and fills the field in automatically as soon as the SMS arrives, provided the message follows a standardized format that includes the site's domain. The two mechanisms complement each other: the WebOTP API for the smoothest experience where it's supported, plain autocomplete as a fallback everywhere else.

The fallback for non-mobile numbers

A landline can't receive an SMS. Simply blocking these visitors means losing legitimate leads in sectors where landlines remain common — certain professional services, trades, older age groups. Offering verification by automated voice call — the service reads the code aloud instead of texting it — covers this case without complicating the interface: a single "receive by call" button, shown after a first failed attempt or when a landline number is detected, is enough.

Mistakes to avoid

• Not rate-limiting the number of codes sent — without a limit per phone number and per IP, the form becomes an open door to SMS pumping (bulk codes sent to premium-rate numbers at the site's expense); a honeypot combined with strict server-side throttling is essential before shipping this to production.
• Underestimating the per-SMS cost at scale — a few cents per send looks negligible until an ad campaign multiplies the number of forms submitted; that cost should be weighed against the real cost of a missed appointment, not ignored by default.
• Verifying before, never after, the sales call — the point of OTP disappears if it happens after a rep has already tried reaching the number; verification needs to gate the form submission itself.
• Storing the verified number like any other personal data — verification changes nothing about GDPR retention obligations or the consent required to recontact someone.

The lighter alternative: validate without verifying

On a higher-volume form, or one with a lower average deal size, OTP can be overkill. Strict format validation with a library like libphonenumber-js — which recognizes valid country codes and lengths rather than a plain digit-count regex — already filters out most grossly wrong entries, with no added step. Paired with outbound call tracking to measure the real rate of unreachable numbers, this approach gives a data-backed basis to decide, later, whether moving to OTP is actually justified on that specific page rather than adopted on principle.

On LanderKit's Local Agency & Trade and Real Estate — Valuation templates, the quote-request form ships as source code: adding an OTP verification route, or simply tightening validation with libphonenumber-js, happens directly in the component, with no third-party plugin to depend on. For multi-step forms, phone verification naturally fits as the last step before submission — see our guide on multi-step forms for structuring it without breaking the perceived progress.