Collecting Phone Numbers for SMS: Methods and Consent

TL;DR. An SMS programme dies at the list, not at the message. A number collected without traceable consent is a number that exposes the brand and does not convert. What works is two-step collection : email first, number later, and systematically retaining the proof of consent, not just the number.

The temptation is to treat number collection like email collection, with the same forms and incentives. That is wrong on two counts: the legal framework is stricter, and visitor behaviour is not the same.

Giving a phone number is a bigger commitment than giving an email address. The form has to account for that.

What consent must contain

For prospecting to individuals, consent must be free, specific, informed and unambiguous. In practice, at a capture point, that means four visible elements:

• a dedicated SMS tick box, separate from terms acceptance and newsletter signup;
• the stated purpose: what the number will be used for;
• the controller's identity;
• a mention of how to opt out.

The regulatory detail for France is in SMS and GDPR in France.

What brands forget most: the proof matters more than the number itself. Date, time, capture page, and the exact wording displayed at the moment of consent. Without that traceability the list is unusable, whatever its real provenance.

Capture points that work

Two-step collection

By far the most effective method. The pop-up asks for the email, the relationship opens, and the number is requested later : on the thank-you page, in the first welcome email, or after the first order.

It produces a smaller list than simultaneous collection, and a markedly more engaged one. Form completion drops significantly as soon as a phone field is added, and numbers obtained under pressure are the ones that opt out fastest.

Pop-up mechanics are covered in email pop-up and signup form strategy.

Checkout

The best moment in the whole journey: the customer is already entering their number for delivery. A dedicated box beneath the field, with clear wording.

Note that a number entered for delivery is not consented for prospecting. The box must be explicit and unticked, or the consent does not stand.

Back in stock

The most favourable context available. The customer wants to be told, availability is time-limited, and SMS is objectively the right channel. Opt-in rates here are the highest on the site.

The loyalty programme

If you run one, the number is naturally legitimate there. Marketing consent still has to be collected separately.

Point of sale

Effective but legally fragile: proof of consent is hard to establish on a paper form or a till entry. A double opt-in text sent immediately afterwards secures the capture.

The two-step collection, and why it beats a single form

Asking for an address and a number in the same field set lowers completion on both. The pattern that holds up is sequential: capture the email first, then request the number later, at a moment when the reason is obvious.

Three moments work particularly well. The thank-you page after an order, where the number reads as delivery tracking rather than marketing. The first welcome email, once the relationship exists. And a back-in-stock request, where the whole point is being told quickly.

Each of these gives you something a generic form never does: a stated purpose the person actually wanted. That is also what makes the consent wording easy to write, and easy to defend later.

What makes a list unusable

Importing without traceability. A file recovered from an old tool, with no date or capture point, cannot be used. The only clean exit is a re-permission campaign: one message asking for confirmation, then deleting everyone who did not reply. It costs volume once and removes the risk for good.

The pre-ticked box. It is not a positive act.

Bundled consent. A single box covering terms, newsletter and SMS covers none of the three.

Renting or buying a list. There is no situation in which this produces a viable programme.

Reusing a delivery number. Covered above, and the most common error because it seems harmless.

What to promise at collection

Whatever you promise here sets the ceiling on your sending plan, so promise something you can hold for a year rather than something that reads well on the form.

The incentive has to match what the contact will receive. Recruiting with "get our exclusive offers" and then sending three messages a week produces an opt-out by the second.

Stating the frequency at collection improves list quality. "Two messages a month maximum" reduces signups and increases how long each one lasts.

What to archive, and why it matters later

Consent is the proof attached to the number, not the number itself. Four things need storing alongside each contact: the date, the capture point, the exact wording displayed at that moment, and the version of that text.

The wording is the item most often lost. A form redesign that overwrites the previous copy without archiving it removes the evidence for every contact captured before the change, including contacts you are still mailing. The fix is cheap up front and impossible retroactively.

One consequence worth stating plainly: consent is specific to a channel. An address collected for the newsletter does not open SMS, and a number collected for SMS does not open WhatsApp. Each channel needs its own capture, its own wording, and its own timestamp.

Measuring collection quality

Three metrics, by capture point:

Opt-in rate per capture point, which says which one actually works.

30-day opt-out rate by capture cohort. The key metric: it reveals which capture points recruit contacts who will not stay. A high-volume, high-churn capture point destroys value.

90-day revenue by cohort, which lets you arbitrate volume against quality.

FAQ

Can we text people who only gave us an email address?

No. Consent is specific to a channel: an address collected for the newsletter does not open SMS, and a number collected for SMS does not open WhatsApp. Each channel needs its own capture point, its own wording and its own timestamp.

What exactly should we store as proof?

Four items per contact: the date, the capture point, the exact wording shown at that moment, and the version of that text. The wording is the one most often lost in a form redesign, and it cannot be reconstructed afterwards.

How do we collect phone numbers legally?

Through a positive act, on a form separate from terms, with purpose and controller stated : and by retaining proof of consent.

Should we ask for the number at the same time as the email?

Rarely. Two-step collection produces a smaller, more engaged list.

Is double opt-in required for SMS?

Not universally, but recommended whenever provenance could be questioned.

Can we use numbers collected for delivery?

Not for prospecting: the purpose of the consent is different.

What SMS list size should we aim for?

Opt-in quality matters more than size.

Going further

Technical setup is in Klaviyo SMS setup and opt-in flows, and the channel framework in SMS marketing: definition and examples.

For full scoping, see our SMS marketing agency.

Review your email and CRM stack with Deliver