A phone number and a short text message can make sign-in feel simple. Customers do not need to remember another password, and many already have a mobile phone nearby. SMS one-time passwords, or OTPs, can therefore be a practical option for some business applications. The convenience comes with security, reliability, and cost trade-offs that should be understood before making phone login the only route into an account.
Why teams choose SMS
The flow is easy to explain: enter a number, receive a code, and type it into the application. It can work on inexpensive devices without a special authenticator app. A phone number can also be useful for service notifications, provided the customer has explicitly agreed to that use. For a low-risk service with occasional visits, this may remove the friction of password creation and recovery. It is particularly helpful when the audience expects phone-based contact and does not consistently use email. Still, possession of a phone number should be treated as a temporary authentication factor rather than a permanent identity.
Security limits
SMS messages are not end-to-end proof that the intended person is present. Number reassignment, SIM-swap fraud, carrier account compromise, malware, and phishing can all undermine the flow. A scammer may persuade a user to read a code aloud or enter it into a fake page. Codes should expire quickly, be single use, and be bound to a specific sign-in attempt. Limit both message requests and verification guesses by account, number, device, and network signals where feasible. Avoid disclosing whether a phone number exists in the system through different response messages. For high-risk administrator actions, use a stronger factor such as a passkey or security key.
Reliability and delivery
SMS delivery can be delayed or fail because of carrier filtering, roaming, regional restrictions, or an incorrect number. A user who cannot receive a message needs a defined fallback. Let them request a new code after a sensible delay, but prevent rapid retries from creating spam or expense. The UI should show the destination in a privacy-preserving way and explain how long the code remains valid. Observe delivery success by country and provider, without storing the full code in logs. If the application serves multiple regions, test the flow with local carriers before committing to SMS as its only sign-in method.
Cost and abuse
Every outbound message has a cost, and automated attackers can create expensive traffic by repeatedly requesting codes to premium or foreign destinations. Set spending limits, monitor unusual country and prefix patterns, and consider whether registration should require another signal before sending. A simple rate limit alone may not stop distributed abuse, but it establishes a baseline. The team should know who investigates sudden messaging spikes and how to temporarily restrict affected routes without locking out all customers. For an SMB, predictable monthly cost matters as much as the smooth sign-in screen.
Account lifecycle
Phone numbers change. Users may leave a company and lose a business number, and mobile operators eventually recycle numbers. Require a recent strong session before changing the registered number, and notify the old channel when possible. Do not automatically transfer account ownership to whoever now receives a code for an old number. Define an account recovery path with human review or a second factor for sensitive accounts. If phone numbers are personal data, collect only what the service needs and follow the applicable retention and consent rules. A number can be a login identifier, but it should not silently become an authorization rule.
Making a balanced choice
SMS OTP is useful when broad device reach and low learning effort are priorities. It is less suitable as the sole protection for privileged or financially sensitive workflows. Offer passkeys or another stronger method where practical, and require a step-up check for changing billing, exporting customer data, or managing other users. Measure sign-in completion, delivery failures, support tickets, abuse, and message spend after launch. The right design is the one that keeps legitimate customers moving while giving the business a realistic way to manage its security and operating costs.