How to Reduce OTP and SMS Verification Fraud
Updated July 26, 2026 · 7 min read
One-time-passcode flows feel secure, but they're a soft target. Attackers use disposable numbers to farm accounts, and abuse the send itself to pump up SMS traffic on routes they profit from. Both attacks start with the phone number — so both can be blunted before you ever send a code.
The two costs of OTP abuse
The first cost is fake accounts. Cheap, throwaway numbers let one actor create thousands of accounts to farm promos, referrals, and free trials.
The second is SMS pumping (also called traffic pumping or toll fraud): attackers trigger huge volumes of OTP sends to numbers on premium routes, collecting a share of the inflated messaging revenue. You pay for every message and get nothing back.
Screen the number before you send
Most of this is preventable at the moment a number is submitted, before the OTP goes out. A quick liveliness and risk check lets you decide whether the send is even worth making:
- Reject disposable and temporary numbers built to receive one code and vanish.
- Weigh VoIP as a risk signal rather than trusting it like an established mobile.
- Confirm the number is live and reachable so you don't pay to text a dead line.
- Watch for surges to unusual number ranges — a hallmark of pumping.
Breaking SMS pumping specifically
Pumping relies on volume to numbers the attacker benefits from. Because those numbers are often unreachable, freshly issued, or clustered in specific ranges, screening for liveliness and risk before the send removes the payout. If the message never goes out, there's nothing to pump.
Pair per-number screening with basic rate limiting so a single actor can't drive thousands of sends regardless of which numbers they use.
Design the flow around risk
The aim is to keep verification invisible for legitimate users and expensive for attackers. A risk-tiered flow does that:
- Low-risk, established mobile number → send the OTP as normal.
- Elevated risk (VoIP, new, unusual range) → step up or throttle before sending.
- Known disposable or high-risk → block or route to an alternate verification method.
Key takeaways
- OTP abuse costs you twice: fake accounts and paid-for SMS pumping.
- A liveliness + risk check before the send stops both at the cheapest point.
- Screening plus rate limiting removes the payout that makes pumping worthwhile.
- Route by risk so real users breeze through and attackers hit friction.
Key terms
Frequently asked
What is SMS pumping?+
SMS pumping (or traffic pumping) is a scheme where fraudsters trigger large volumes of verification texts to numbers on routes they profit from, inflating your messaging bill while providing no real signups. Screening numbers before sending removes the numbers they rely on.
Won't screening numbers add friction for real users?+
Not if you route by risk. The vast majority of legitimate signups use established mobile numbers that pass instantly; only disposable or high-risk numbers get extra verification, so genuine users rarely notice.