Everything worth knowing, nothing that isn't.
How number verification works
The classic flow: your app requests a one-time passcode, the platform generates it and delivers it to the number over a channel (SMS, voice, WhatsApp), and the user types it back to prove possession. Newer methods skip the code entirely — flash call verification reads the passcode from an unanswered call's caller ID, and silent network authentication verifies the SIM against the operator over a data session invisibly. See /products/otp, /products/myotp, and /answers/what-is-otp.
The verification methods
SMS OTP (universal reach, most common); voice OTP (reaches landlines and SMS-degraded markets, read aloud); WhatsApp OTP (rich, cheap where the app is installed); flash call (fast, cheap, no message body, Android-friendly); and silent network authentication (zero user friction, strongest against interception). A robust verification service fails over between them by country and delivery result. See /answers/what-is-2fa and /answers/what-is-flash-call-verification.
Silent network authentication and GSMA Open Gateway
Silent network authentication (SNA) opens a data connection that the mobile operator authenticates against the SIM, confirming possession of the number with nothing for the user to type and nothing an attacker can phish or intercept. It is exposed through GSMA Open Gateway and the CAMARA Number Verification API, and it is the lowest-friction, highest-assurance option where operator coverage exists. See /answers/silent-network-authentication and /answers/gsma-open-gateway.
The fraud problem: AIT and SMS pumping
OTP traffic is the prime target for artificially inflated traffic (AIT), also called SMS pumping: fraudsters trigger floods of OTP sends to premium-rate number ranges they profit from, draining your verification budget. Defences include pre-send HLR/number-range validation, per-number and per-IP rate limits, and shifting high-risk geographies to flash call or silent auth. See /answers/what-is-ait and /answers/what-is-sms-pumping.
Benefits and choosing a method
Number verification stops account takeover, bot signups, and payment fraud while proving a reachable, real customer. Choose by market: SMS for universal reach, silent auth or flash call to cut cost and friction where covered, voice as a fallback, and always validate numbers first to blunt AIT. BroadNet's verification stack routes across all of these. See /products/hlr-lookup and /operators/flash-call.
Which number verification method is most secure?
Silent network authentication is the strongest: because it checks the SIM against the operator over the data plane, there is no code to phish, intercept over SS7, or social-engineer out of a user. Flash call is next. Plain SMS OTP is the most universal but the most attackable, so pair it with number validation and rate limits. The practical answer is a policy that fails over between methods by risk and coverage. See /answers/what-is-2fa.