# How to Improve SMS Opt-In Rates with Better Phone Data > Higher SMS opt-in rates start with validating phone numbers at collection. Learn how phone data improves subscriber quality and campaign performance. Published 2026-01-15, updated 2026-10-03 by Marcus Whelan. Canonical: https://checkthatphone.com/blog/sms-opt-in-rates-phone-validation You can run a flawless double opt-in flow and still bleed subscribers, because the confirmation text only works if the number can receive it. It's easy to obsess over button copy and form placement while quietly losing part of every cohort to numbers that were never going to get the message: landlines, typos, dead lines. Collecting the number is half the job. Making sure it can actually receive an SMS is the half people skip. Validating at the point of collection closes that gap. Every number that enters your opt-in flow gets checked for whether it can take an SMS, which lifts your conversion from number collected to confirmed subscriber. ## Why opt-in rates underperform Most teams define opt-in rate as the share of users who hand over a number and then confirm via double opt-in, usually by replying YES or tapping a link in a confirmation text. A few things quietly drag that number down: Invalid numbers from typos, fakes, and bad formatting mean the opt-in message never sends. Landlines, which people enter more often than you would expect, mostly can't receive SMS (about 7% of landlines we check are SMS-capable). Disconnected lines fail silently with no error you will notice. Some VoIP numbers do not reliably receive short-code or long-code SMS. And a confirmation sent at 2 a.m. local time gets ignored and forgotten by morning. Every one of these is a lost subscriber: someone who meant to sign up and never saw the confirmation. ## How validation fixes the funnel ### 1. Catch invalid numbers at entry Check the number in real time when the user submits the form. If it is malformed, invalid, or recently deactivated, show an inline error and ask them to fix it. That catches typos and junk before you waste an opt-in send. ### 2. Filter out non-SMS numbers Use [line-type detection](/line-type-lookup-api) to confirm the number can take a text. The simplest check is `smsEligible`: it's `"true"` for an active mobile, and for a landline confirmed SMS-capable if you turn on the landline SMS lookup. If someone enters an ordinary landline, ask for a mobile number on the spot. That is a far better experience than firing off a text that silently never arrives and leaving them to wonder why nothing happened. ([Why your SMS campaign is silently dropping landlines](/blog/why-your-sms-campaign-is-silently-dropping-landlines-and-how-to-detect-text-enabled-ones) covers the landline side in detail.) ### 3. Time the message for waking hours The API returns timezone data (`timezone` and `tzOffset`), so you can schedule the confirmation for a reasonable local hour. A text at noon is far more likely to get a reply than one that lands overnight, and this change is easy to ship. ### 4. Route by carrier Carriers handle opt-in traffic differently. Knowing the carrier helps you pick the right sender type (short code, long code, or toll-free) and spot carrier-specific filtering before it eats your delivery rate. If you send from long codes, [10DLC registration](/blog/10dlc-registration-phone-validation) is the other half of that. ## Measuring the improvement Track these before and after you turn validation on: Opt-in message delivery rate, the share of confirmations that actually deliver. Double opt-in completion rate, the share of delivered confirmations that become confirmed subscribers. List growth rate, your net new confirmed subscribers per period. And cost per subscriber, total acquisition cost over confirmed subscribers. How much validation moves these numbers depends on how dirty your inbound data was to begin with. The gain comes almost entirely from stopping sends to non-mobile and invalid numbers before they fail, so measure your own baseline first. For the same idea applied to account signups, see [verifying phone numbers at signup](/blog/verify-phone-numbers-signup-conversion). ## Fitting it into common platforms Validation slots in front of the SMS tools you already run. With Klaviyo, Attentive, or Postscript, check the number before it enters the platform. On custom forms, call the [CheckThatPhone API](/docs) at submission, ahead of the opt-in trigger. On Shopify or WooCommerce, validate the phone field at checkout before the customer joins your SMS list. ## Getting started Better opt-in rates start with better phone data, full stop. Validate at the point of collection and every confirmation goes to a real, SMS-capable device, which turns more signups into actual subscribers. [Read the API documentation](/docs) to wire validation into your opt-in flow, or [explore pricing plans](/pricing) to pick a tier. Start by validating one form, watch your delivery rate move, then roll it out everywhere you collect numbers. --- CheckThatPhone is a phone validation API for US and Canada numbers: https://checkthatphone.com (docs: https://checkthatphone.com/docs.md, pricing: https://checkthatphone.com/pricing.md).