Developers
Test phone numbers by country: fictional ranges for developers
Fictional phone number ranges that regulators reserve in the UK, US, Canada, Australia, Germany and France, how libphonenumber treats them, and safe OTP tests.
11 min read
Articles tagged “OTP”
20 articles.
Research
OTP cost by country: WhatsApp authentication rates, SMS cost drivers and the cost of a wasted code
WhatsApp authentication rates for 44 markets (Meta, October 2026), why SMS OTP prices differ by country, and when a number check costs less than a wasted code.
14 min read
Fraud prevention
SMS pumping by country: which destinations to allow, gate or block
No public list ranks SMS pumping countries. How to sort destinations into allow, gate and block from published sources and your data, and which ranges to block.
12 min read
Fraud prevention
Silent authentication and flash call, explained
How silent network authentication (CAMARA Number Verification) and flash-call verification work, their coverage limits, and where pre-checks still help.
6 min read
Fraud prevention
OTP bombing: how to protect your send-code endpoint
OTP bombing floods a phone with codes from your send-code endpoint. A rate-limit recipe by number, IP, device and prefix, with pre-send checks.
7 min read
Guides
Phone validation vs phone verification: what's the difference?
Phone validation checks the number; verification proves the user holds it. What each one stops, what they cost, and how to combine them in one flow.
7 min read
Deliverability
"This number cannot receive SMS": what the error means and how to check a number before you send
Why a number is not SMS-capable (landline, toll-free, VoIP, invalid, opted out), what the API error codes mean, and how to check a number before you send.
14 min read
Fraud prevention
Account takeover via phone-number change: a checklist for the change-number flow
Attackers change an account's phone number before resetting everything else. A checklist to secure the change-number flow without locking out real users.
7 min read
Deliverability
WhatsApp Business API pricing in 2026: per-message rates, the October changes and India's rupee deadline
WhatsApp Business API pricing 2026: Meta charges per delivered template by category and country (OTP $0.0008 to $0.0550). October changes, INR deadline.
11 min read
Fraud prevention
Toll fraud in voice OTP: why providers block codes as suspected toll fraud
Why identity and messaging providers block voice and SMS codes as suspected toll fraud, what triggers it, and a pre-call check recipe for developers.
7 min read
Deliverability
SMS delivery reports (DLR): what each status really means
What an SMS delivery report is, what DELIVRD, UNDELIV, EXPIRED and REJECTD mean, why 'delivered' isn't proof of reading, and why DLRs go missing or wrong.
7 min read
Guides
HLR lookup results explained: absent subscriber, unknown subscriber and what to do next
Absent subscriber: the number exists but the phone is off or out of reach. Unknown subscriber: the number is not assigned. Each HLR result and what to do next.
10 min read
Fraud prevention
How to build a phone number risk score from carrier, line type and reputation signals
Turn carrier, line type, network status and reputation signals into a phone number risk score with three outcomes, reason codes and calibrated thresholds.
8 min read
Deliverability
When does an HLR lookup pay for itself? Break-even math for SMS and OTP costs
A simple cost model for pre-send checks: when an HLR, MNP or carrier lookup costs less than the SMS it saves, with formulas and a worked table.
9 min read
Fraud prevention
IRSF: international revenue share fraud, explained
IRSF is fraud where attackers trigger calls or texts to numbers they profit from. No free IRSF number database exists; use range lists and line-type checks.
8 min read
Fraud prevention
OTP fraud prevention: the checks to run before you send a code
Before sending an OTP, look up the number: number type, line type and carrier, HLR status, porting. Which APIs offer it, what to compare, a pre-send flow.
10 min read
Deliverability
WhatsApp OTP vs SMS OTP: cost per verified user, with a calculator
WhatsApp OTP pricing in 2026: Meta charges $0.0008 to $0.0550 per delivered authentication message, by country. Rates vs SMS and a cost-per-user calculator.
9 min read
Fraud prevention
SIM swap fraud: the signals to check before you trust an SMS code
What a SIM swap is, which SIM-swap data exists and who can get it, and which signals to combine before you trust an SMS code when that data is missing.
9 min read
Deliverability
Why SMS messages aren't delivered: a cause tree with fixes
A cause tree for undelivered SMS: invalid numbers, unreachable phones, wrong line types, carrier filtering and handsets, with a fix for each.
7 min read
Fraud prevention
SMS pumping: how it works and how to stop it
How SMS pumping (artificially inflated traffic) drains OTP budgets, the signals that give it away, and a layered checklist to stop it before you pay.
8 min read
