Definition
Flash call verification confirms a phone number by placing a very short call that is dropped before it is answered; the last digits of the calling number act as the one-time code, which the app reads automatically or the user enters.
The verification service calls the user from a number drawn from a pool. The call is cut after one ring or less, so the user normally does not answer it. The app compares the caller ID it receives with the expected number, or asks the user to type the last few digits shown in the missed-call notification.
Automatic detection depends on the app being able to see incoming caller IDs. That is possible on Android with the right permissions but not on iOS, where third-party apps cannot read the call log, so iPhone users usually type the digits manually or fall back to SMS or WhatsApp.
Flash calls are cheap to deliver and hard to intercept with SMS-based attacks, but they are controversial with operators. Because the call is never answered, the terminating network earns little or nothing, and some operators classify flash calls as A2P traffic, apply specific termination charges or block them. Availability and cost therefore vary by country and network and should be checked before relying on the method.
Like any number-verification method, flash calls can be abused to generate traffic to high-cost ranges. Apply rate limits and country controls, and keep SMS, WhatsApp or voice OTP as fallback channels.
Example
Missed call from +971 4 555 1234 — the app takes "1234" as the verification code.