# How to Verify Phone Numbers at Signup Without Hurting Conversion > Real-time phone verification at signup improves data quality without adding friction. Learn implementation strategies that protect conversions. Published 2025-07-05, updated 2026-10-03 by Marcus Whelan. Canonical: https://checkthatphone.com/blog/verify-phone-numbers-signup-conversion Phone verification at signup is a balancing act, and most teams fall off one side or the other. Make it too heavy and legitimate users abandon the form. Skip it and your database fills with typos, fake numbers, and unreachable contacts that quietly cost you money for months. The version that actually works runs in the background: real-time validation that catches bad data without the user ever feeling it. ## Why the signup field matters more than it looks A phone number captured at registration doesn't stay put. It flows into your marketing lists, your 2FA system, your sales queue, and your support tools. A bad number entered once compounds across every one of them: - Marketing: SMS to invalid numbers wastes budget and drags down your sender reputation - 2FA: OTP codes to disconnected numbers produce frustrated users and support tickets - Sales: reps lose time on numbers that ring out or reach the wrong person - Analytics: bad contacts distort your funnel and inflate acquisition cost Catching the problem at the point of collection is dramatically cheaper than cleaning it out of four systems later. ## Validating without adding friction The goal is to check the number without making the user work for it. A few patterns that work: ### Validate on blur or submit Call the API when the user tabs out of the phone field or hits submit, not on every keystroke. You get a complete number to check and you avoid laggy, half-typed validation while they're still entering digits. ### Show inline feedback If the number is invalid, undeliverable, or a landline when you need mobile, show a plain inline error: "Please enter a valid mobile number." It reads like normal form validation, which is exactly what users expect. ### Accept VoIP selectively Not every VoIP number is fraud. Google Voice and business VoIP lines are legitimate, and blanket-blocking VoIP will reject real users. Use the carrier data to make a finer call: `dipCarrierSubType` marks VoIP as `IPES` and `dipCarrier` names the provider, so you can flag the providers you've seen abused and let the rest through. [Vetting inbound leads](/blog/vetting-inbound-leads-the-phone-signals-that-flag-obvious-fraud) shows a simple scoring model for this. ### Don't make validation the only gate Phone validation should inform your signup logic, not be the entire decision. Pair it with email verification, CAPTCHA, and behavioral signals so you catch fraud in layers instead of leaning on one check that's easy to slip past. ## What the API hands back at signup Call CheckThatPhone's API with a number and you get: - Line type: `dipCarrierType` (mobile, landline, invalid) from a live carrier dip, with VoIP identified by `dipCarrierSubType` - Deliverability: `deliverable` and `smsEligible`, whether the number can take an SMS - Carrier: `dipCarrier`, the operating carrier, and `dipCarrierSubType`, the network type - Deactivation: `deactivationDate`, set when the carrier deactivated the number in the last 45 days - Action recommendation: `action` is `"send"`, `"unsubscribe"`, or `"error"` (a malformed request), with `reason` explaining an `"unsubscribe"` [See the full response format](/docs) to map these fields into your signup logic. ## Measuring whether it worked Capture these before you turn validation on, then watch them after: - Invalid number rate: share of signups with bad phone data - 2FA delivery rate: share of OTP codes that actually land (see [improving 2FA with phone validation](/blog/improve-2fa-phone-validation)) - Sales contact rate: share of leads who answer or reply - Support tickets: volume of "I never got my code" complaints If validation is doing its job, all four move in the right direction. If you also collect SMS consent at signup, [improving SMS opt-in rates](/blog/sms-opt-in-rates-phone-validation) covers that part of the flow. ## Next step You don't need OTP codes and extra screens to get clean phone data. Real-time API validation catches bad numbers silently and keeps the signup flow fast. [Read the API documentation](/docs) to add it to your registration forms, or [explore pricing plans](/pricing) and start on the free tier. --- 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).