# Improve Two-Factor Authentication (2FA) with Real-Time Phone Validation > Demonstrates how ensuring authentication codes reach valid, reachable mobile devices strengthens 2FA security. Published 2025-04-30, updated 2026-10-03 by Marcus Whelan. Canonical: https://checkthatphone.com/blog/improve-2fa-phone-validation Most 2FA flows eventually hit the same wall: a code that never arrives. The user swears they entered the right number, your logs say it sent, and support is now stuck in a loop neither side can win. Often the number was a landline, or disconnected, or a throwaway VoIP line an attacker spun up to farm accounts. The fix is to check the number before you spend an SMS on it. Confirm it's real, active, and able to receive a text, and a whole category of problems disappears before it starts. ## The problem with "blind" OTPs Firing one-time passcodes at unverified numbers creates trouble on a few fronts at once: - Delivery failures: the number is disconnected, invalid, or a landline that can't take SMS. - Delays and re-sends: users who never got the code request three more. - Support load: those same users land in your help desk queue. - Security gaps: attackers lean on VoIP numbers to spin up accounts in bulk. A number can pass a format check and still be unreachable or wrong for SMS. Valid-looking is not the same as deliverable. ## What validation checks before you send A real-time check evaluates each number and hands back the signals that actually matter: - Line type: mobile, landline, or VoIP. OTPs belong on mobile numbers. ([Line type lookup](/line-type-lookup-api) explains the codes, including `IPES` for VoIP.) - Carrier: helps you spot risky or disposable networks. - Deliverability: a single `deliverable` / `action` summary of line type, blacklist, and recent deactivation, so you know whether the number is worth a code. - Deactivation: `deactivationDate` flags numbers deactivated in the last 45 days, which stops codes going to dead lines. - Portability: `dipPorted` says whether the number has been ported away from its original carrier. It carries no date, so on its own it can't tell you a port was recent. You decide what to do with that: send the code, show an error, or ask for another method. CheckThatPhone's API returns all of these in one request. [See the full documentation](/docs) for integration details. ## How it works in practice A validation-enhanced 2FA flow is short: 1. User enters their phone number to enable 2FA. 2. Your app sends the number to the [CheckThatPhone validation API](/docs). 3. The API responds with validity, line type, and deliverability status. 4. If anything fails, show an inline message or fall back to email-based 2FA. It's one real-time API call, and you keep full control over whether to proceed. The same check at signup also keeps throwaway numbers out of your user base; see [phone validation for onboarding fraud prevention](/blog/phone-validation-a-critical-tool-for-customer-onboarding-fraud-prevention). ## A second use case: spotting port-outs Port-out fraud is when an attacker moves a victim's number to a carrier they control, so the one-time codes go to them. Carrier data can catch that. Store the carrier details from an earlier lookup (`dipCarrier`, `dipOcn`, `dipLrn`), and if a fresh lookup before a sensitive action shows a different carrier, the number has been ported since you last checked. That's worth a step-up challenge before you send a code. A `deactivationDate` that appeared since your last check is a similar warning. A classic SIM swap is a different attack. The attacker talks the victim's own carrier into moving the number to a new SIM, the carrier doesn't change, and nothing in carrier or porting data shows it. A lookup won't detect a SIM swap, so if that's your threat model you need a carrier SIM-swap signal or a factor that doesn't depend on SMS. Tracking carrier changes earns its keep for: - Banking and fintech: catching port-out attempts before a code goes out ([KYC phone verification](/use-cases/kyc-phone-verification) covers the onboarding side). - High-security accounts: forcing re-auth when the number's carrier changed. - Regulated industries: cutting risk without piling on friction. ## Getting started Your 2FA is only as strong as the number receiving the code. Validating in real time keeps OTPs flowing to active, SMS-capable mobiles instead of dead lines or VoIP throwaways, and it gives you a signal to challenge suspicious changes. Adding it to an existing auth flow is a few lines and one API call; if you also collect the number at signup, [verifying phone numbers without hurting conversion](/blog/verify-phone-numbers-signup-conversion) covers the form side. [Explore the API documentation](/docs) to see how it works, or [check out pricing plans](/pricing) to find the right fit for your volume. --- 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).