SIM Swap Detection API: What It Checks and When to Use It

A customer enters a mobile number, receives a one-time password and completes a sensitive action.

The journey may look secure. But if the SIM linked to that number changed shortly before the OTP was requested, the code could be reaching somebody different from the person the business expects.

A SIM swap detection API provides live carrier information about recent SIM activity before a business relies on the mobile channel. It can add useful context during account recovery, password resets, payment-detail changes and other journeys where possession of a number carries significant trust.

What is a SIM swap?

A SIM swap occurs when a mobile number becomes associated with a new SIM. This can happen legitimately when a customer replaces a lost or damaged SIM, activates an eSIM, upgrades a device or moves to another network while retaining the number.

It can also be abused. In a fraudulent SIM swap, an attacker attempts to move a victim's number to a SIM they control. If successful, calls and text messages intended for the customer may be redirected to the attacker.

That can weaken an authentication process that relies mainly on SMS one-time passwords. The important qualification is that recent SIM activity is a risk signal, not proof of fraud.

What does a SIM swap detection API check?

A SIM swap API checks recent SIM activity associated with the mobile number used in the customer journey. Depending on the available carrier response, it may indicate:

• Whether a SIM change occurred within a defined period
• When the latest available SIM change occurred
• How recent the change was
• What time window or response status applied

This lets the business compare the timing of the SIM activity with the action being attempted. The signal becomes more meaningful when it appears immediately before an account recovery request, payment change or another sensitive event.

Why an OTP is not enough on its own

An OTP shows that somebody can receive a code sent to the number at that moment. It does not explain whether the SIM receiving that code changed shortly before the request.

An attacker who has taken control of the number may be able to receive the same OTP intended to protect the account. A SIM activity check therefore adds another question before the business trusts the mobile channel:

The answer should be considered alongside the customer's established account history, identity evidence, behaviour and other risk signals. Mobile KYC answers a different question by checking whether submitted identity details match carrier-held information associated with the number.

When should a SIM swap check happen?

The most useful time to run the check is immediately before the business relies on the number for a sensitive action, including:

1. Account recovery
2. Password or security-detail resets
3. Adding a new payee
4. Changing payment or withdrawal details
5. Updating contact information
6. Approving a higher-risk transaction
7. Accessing an existing policy or account

The signal should be recent enough to be relevant to the action. If recent SIM activity is identified, the business can use its own rules to request more evidence, pause the action or route it for review rather than declining the customer automatically.

A simple SIM swap detection flow

1. The customer supplies or uses their registered mobile number.
2. The business requests a live SIM activity check.
3. The mobile network returns the available SIM change information.
4. The business combines the result with its own risk rules and other evidence.
5. The journey continues normally or moves to proportionate step-up verification.

The TDS Risk quickstart guide shows developers how to run the first carrier-derived API check.

Does recent SIM activity mean fraud?

No. Customers replace damaged SIMs, change devices, move networks and activate eSIMs for legitimate reasons.

The value comes from timing and context. A SIM change shortly before a password reset, new-payee request or withdrawal-detail change may justify more evidence than the same signal during a low-risk journey.

This is why the strongest implementation uses graduated actions and explainable reason codes rather than one automatic decline rule.

SIM swap detection API vs mobile number risk score

A SIM swap detection API answers a focused question about recent SIM activity.

A mobile number risk score considers a wider group of carrier-derived signals. These may include recent SIM activity, number tenure, line type, ownership stability, carrier information and network consistency.

The two approaches are related but not identical. A focused SIM check is valuable immediately before an OTP, account reset or transaction. A broader score can be used earlier at Step 0, before a business pays for KYC, credit searches or manual review.

TDS Risk Trust Score brings the available mobile signals together into an explainable score from 0 to 1,000, supported by individual indicators and reason codes.

What should a good SIM swap API response include?

A useful response should provide:

• A clear recent-SIM-activity result
• The applicable time window or available date information
• A unique request identifier
• The time the check was performed
• A consistent response structure
• Clear reason codes when the result contributes to a wider score
• An audit record connected to the check

The business should be able to explain which signal it received and which rule it applied next. That matters for operational review, customer support, compliance and governance. TDS Risk's platform controls are detailed on its security and compliance page.

Where can SIM swap detection be used?

Banking and fintech

Check recent SIM activity before account recovery, new payees, payment-detail changes and other sensitive actions. The fintech and payments use case shows how the signal can fit alongside existing controls.

Consumer lending

Add a live mobile signal before progressing an application, changing payout details or resetting access.

Insurance

Check the mobile number before accepting changes to contact, payment or policy information and during selected account-recovery journeys.

iGaming

Use recent SIM activity as one input during account recovery, withdrawal-detail changes and other higher-risk actions.

Digital account creation

Assess the number before relying on SMS verification to open or recover an account.

Call centres

Give an agent additional context when a caller requests a sensitive change that relies on the registered mobile number.

How TDS Risk Trust Score uses SIM activity

Trust Score combines real-time, carrier-derived mobile signals into an explainable score from 0 to 1,000. Recent SIM activity is one available input alongside signals relating to number tenure, line type, ownership stability and carrier or network information.

TDS Risk operates across Vodafone, EE, Virgin Media O2 and Three and typically returns results in under two seconds. Trust Score costs £0.35 per check excluding VAT, with no contract, minimum volume or setup fee. Prepaid credit does not expire. Current commercial terms are available on the pricing page.

Every check is connected to an audit record, giving the business a referenceable account of the request and returned signals. Businesses considering Step 0 savings can test their own assumptions using the fraud prevention ROI calculator.

Add context before trusting the mobile channel

SMS authentication remains useful, but receipt of a code is not the entire risk picture.

A live SIM activity signal helps a business understand whether the mobile channel changed shortly before a sensitive request. Used with other evidence, it can support a proportionate decision: continue, request further verification or review the action.