SMS opt-in rates directly determine the size and quality of your messaging audience. But collecting phone numbers is only half the equation — if those numbers are invalid, disconnected, or non-mobile, they’ll never receive your opt-in confirmation and never join your subscriber list. Real-time phone validation at the point of collection ensures that every number entering your opt-in flow is capable of receiving SMS, dramatically improving your conversion from “phone number collected” to “confirmed subscriber.”
Why Opt-In Rates Underperform
Most businesses measure opt-in rate as the percentage of users who provide a phone number and confirm their subscription via double opt-in (typically by replying YES or clicking a confirmation link sent via SMS). Several factors suppress this rate:
- Invalid numbers — Typos, fake numbers, and formatting errors prevent the opt-in message from being sent
- Landlines — Users sometimes enter their home or office landline, which can’t receive SMS
- Disconnected lines — Numbers that are no longer active silently fail
- VoIP limitations — Some VoIP numbers don’t reliably receive short code or long code SMS
- Wrong timezone — Opt-in messages sent at 2 a.m. local time get ignored or forgotten
Each of these failures represents a lost subscriber — someone who intended to sign up but never received the confirmation message.
How Phone Validation Fixes the Funnel
1. Catch Invalid Numbers at Entry
Validate the phone number in real time when the user enters it. If the number is malformed, disconnected, or undeliverable, show an inline error prompting them to correct it. This catches typos and fake entries before they waste an opt-in message.
2. Filter Non-SMS Numbers
Use line type detection to ensure the number is mobile. If a user enters a landline, prompt them to provide a mobile number instead. This is a far better experience than sending an SMS that never arrives.
3. Time the Opt-In Message Right
Use timezone data returned by the API to schedule opt-in messages during waking hours. A confirmation text sent at noon local time gets a much higher response rate than one sent overnight.
4. Route by Carrier
Some carriers handle opt-in flows differently. Knowing the carrier helps you choose the right sender type (short code vs. long code vs. toll-free) and avoid carrier-specific filtering issues.
Measuring Improvement
Track these metrics before and after implementing phone validation:
- Opt-in message delivery rate — Percentage of confirmation messages successfully delivered
- Double opt-in completion rate — Percentage of delivered confirmations that result in confirmed subscribers
- List growth rate — Net new confirmed subscribers per period
- Cost per subscriber — Total acquisition cost divided by confirmed subscribers
Businesses that validate phone numbers at collection typically see a 15-30% improvement in opt-in completion rates simply by eliminating sends to non-mobile and invalid numbers.
Integration with Common Platforms
Phone validation works alongside your existing SMS marketing tools:
- Klaviyo / Attentive / Postscript — Validate numbers before they enter your SMS platform
- Custom forms — Call the CheckThatPhone API on form submission before triggering the opt-in flow
- Shopify / WooCommerce — Validate the phone field at checkout before adding the customer to your SMS list
Getting Started
Better SMS opt-in rates start with better phone data. By validating numbers at the point of collection, you ensure every opt-in message reaches a real, SMS-capable mobile device — turning more signups into confirmed subscribers. Read the API documentation to integrate phone validation into your opt-in flow, or explore pricing plans to start improving your subscriber acquisition today.