How should security teams choose between email and phone number authentication for different customer journeys?

Why This Matters for Security Teams

Choosing email or phone number authentication is not just a UX decision. It shapes account recovery risk, fraud exposure, support burden, and the cost of failed delivery. For customer journeys that begin on desktop or involve low-frequency access, email often fits user expectations and is easier to operate. For mobile-first onboarding, phone numbers can feel faster, but they also introduce SMS dependency, regional variability, and SIM-swap risk.

Security teams should treat this as an identity assurance design question, not a channel preference. The wrong choice can create friction where the business needs conversion, or false confidence where the journey actually needs stronger verification. That is especially important because identity signals are often reused across signup, login, and recovery without re-evaluating the threat model. NIST’s control guidance in NIST SP 800-53 Rev 5 Security and Privacy Controls supports channel selection as part of broader access and authentication governance.

In practice, many security teams discover the weaknesses of a chosen channel only after delivery failures, takeover attempts, or account recovery abuse have already started.

How It Works in Practice

The practical decision starts with journey mapping. Email is usually better when the journey is frequent but not time-critical, when users commonly switch devices, or when the organisation wants lower operational cost. Phone number authentication tends to work better when mobile is the dominant device, when users expect quick one-time verification, or when fraud teams need a stronger signal against disposable accounts. Neither option is universally stronger; the right choice depends on how the customer actually enters, proves, and recovers access.

Teams should evaluate four implementation factors:

• Delivery reliability: email may be delayed or filtered, while SMS can fail on roaming, prepaid, or international numbers.

• Attack surface: phone-based flows are exposed to SIM swapping and SMS phishing, while email-based flows depend on mailbox security and inbox takeover resistance.

• Recovery design: whichever channel is primary should not be the only recovery path if that would create lockout or single-point failure.

• Risk tiering: higher-risk actions should not rely on the same channel that was used for casual login if step-up verification is available.

This is where real-world telemetry matters. If email verification is being abused for mass signups, throttling and bot controls may matter more than changing channels. If mobile delivery is unreliable in a target market, phone authentication can become a support issue rather than a security improvement. The broader lesson from NHIMG research on exposed credentials in the DeepSeek breach and Twitter Source Code Breach is that weak identity handling rarely stays isolated to one workflow. These controls tend to break down when customer journeys span multiple regions and carrier networks because SMS delivery, number portability, and fraud patterns vary too much to assume consistent assurance.

Common Variations and Edge Cases

Tighter authentication choice often increases operational overhead, requiring organisations to balance fraud reduction against deliverability, accessibility, and support cost. That tradeoff becomes visible in edge cases where the default channel does not fit the user population.

Current guidance suggests using email for desktop-heavy B2B portals, long-lived accounts, and low-risk signups, while reserving phone number authentication for mobile-first consumer onboarding or journeys where instant verification matters more than channel cost. Best practice is evolving for recovery flows: there is no universal standard for this yet, but many teams avoid relying on the same channel for both primary login and account recovery.

Teams should also account for shared devices, inaccessible inboxes, prepaid SIMs, international users, and regulated environments where stronger identity proofing may be needed. ISO governance guidance in ISO/IEC 27001:2022 Information Security Management supports documenting the rationale for the chosen channel and reviewing it as risk changes. The practical answer is to match the channel to the journey, then test it against failure modes before launch, not after customer complaints start.

Related resources from NHI Mgmt Group

• How should security teams choose between FIDO and certificate-based authentication?
• How should security teams choose between authentication controls and IGA controls?
• How should security teams choose between OAuth flows for different client types?
• How should security teams design authentication beyond login for customer journeys?