SIM tenure

SIM tenure is the length of time the current SIM has been associated with a phone number. Long tenure is weak evidence of stability; very short tenure, hours or days, is strong evidence that something changed, and a high-value action arriving right after a tenure reset deserves step-up verification rather than an SMS code sent to the possibly-hostile line.

Tenure is most useful read as an asymmetry rather than a threshold. A long tenure says little, because a patient attacker waits; a tenure measured in hours says a great deal, because almost no legitimate flow requires a line that changed hands this morning to also approve a high-value action this afternoon.

The asymmetry makes tenure cheap to use correctly and easy to use wrongly. A rule that requires long tenure to permit an action will decline legitimate users who genuinely replaced a handset yesterday, and there are many of them; a rule that treats very short tenure as a reason to ask for stronger proof touches far fewer people and catches the case that matters. The second shape survives a product review.

Tenure is also a derived value rather than a raw one, so the derivation deserves a look. It is computed from the time of the last observed change, which means it inherits the provider's coverage and its reporting lag: a line the provider cannot see has no tenure, not infinite tenure. That distinction is the MVNO opacity problem wearing a different name.

Where tenure is short and the action is high-value, the useful response is a challenge that does not travel over the line, since the line is the thing in question. Sending an SMS OTP to a number that changed hands this morning asks the possibly-hostile party to approve its own request.

Reading tenure as one input among several, rather than as a gate on its own, is what keeps it proportionate.