SIM swap detection

SIM swap detection is a check against live carrier data that reports whether, and how recently, the SIM behind a phone number changed. Performed inside an authentication or recovery flow, it catches the case where a credential is valid but the line behind it changed hands hours earlier, the exact case one-time passcodes cannot see.

Detection quality varies on two axes: freshness (a real-time query versus a nightly batch) and coverage (carriers and MVNOs differ in what they expose). Neither axis is visible in a vendor's headline claim, which is why both belong in procurement questions.

The two axes that decide quality, freshness and coverage, are also the two a vendor's headline number hides. A claim about accuracy describes performance on lines the provider can see, at whatever age its data happens to be, and neither figure is in the claim. Procurement questions that get at the difference are concrete: which operators in this market, real-time query or batch, what is returned for a line you cannot observe, and what is the age of the answer.

The integration question is where the check sits. Run after a code is sent, it reports on an event that has already happened; run before, inside the authorization path, it can stop the code from being sent to a line that changed hands this morning. The second placement is more work because it puts a network call on a latency-sensitive path, and it is the placement that changes outcomes.

Treating a no-answer as a pass is the failure that recurs, because it is invisible in testing: every line the test account uses is observable. Carrying the unknown state through to the rule, and deciding there what it means for this action, is the part worth reviewing by hand. Where the answer is merely old rather than missing, signal freshness decides whether it still counts.

Where a line cannot be observed at all, the honest branch is the one MVNO opacity describes rather than a pass.