VoIP Number Lookup: How to Spot Virtual Phone Numbers at Signup Before They Become Fraud

Introduction

A phone number used to be a strong sign that a real person was behind a new account. Getting a number meant a SIM card, a contract, or at least a trip to a store. Today, anyone can create a virtual phone number in minutes, often for free, and use it to pass SMS verification on your signup form.

That matters because signup forms are now one of the busiest targets online. Okta's research on its Auth0 platform found that attackers hit registration flows at a remarkable rate.

"In 2024, an average of 46% of all registration attempts across the Auth0 platform were identified as signup attacks." Source: Okta Customer Identity Trends Report 2025

This guide explains what VoIP and virtual numbers are, why fraudsters love them, and how to run a practical VoIP number lookup process at signup without turning away good customers.

What VoIP and Virtual Phone Numbers Are

VoIP stands for Voice over Internet Protocol. A VoIP number routes calls and texts over the internet instead of through a traditional mobile SIM or landline. Virtual phone numbers are a broader group: numbers that are not tied to a physical device or SIM card.

There are a few common types:

• Business VoIP numbers from cloud phone systems used by companies and call centers
• App based numbers created inside calling and messaging apps
• Temporary or disposable numbers offered by websites that receive SMS for anyone, often publicly

They all look like normal phone numbers on a form. The difference is how easy they are to create, share, and throw away.

Why Fraudsters Use Virtual Numbers at Signup

For an attacker, a virtual number solves a real problem. Many sites ask for a phone number because it is harder to fake than an email address. Virtual numbers remove that barrier.

Common abuse patterns include:

• SMS verification bypass: receiving one time codes on a number the attacker controls for just a few minutes
• Multi accounting: creating many accounts, each with a fresh number, to look like separate people
• Promo and referral abuse: claiming new user bonuses, free trials, or referral rewards again and again
• Fake reviews and spam: running many accounts to post content or messages at scale

Each of these turns a cheap number into real cost for your business.

The Real Cost of Letting Throwaway Numbers Through

Accepting any number that can receive a code feels safe, but the costs build up quietly:

• Promo budgets drained by repeat signups
• Free trial capacity used by the same few people
• SMS bills for codes sent to numbers that will never become customers
• Polluted user data that weakens your analytics and marketing
• Fake accounts that later commit payment fraud or abuse other users

Greip's SMS fraud use case explains how checking numbers before sending messages helps protect your SMS budget.

Not Every VoIP User Is a Fraudster

Before you block every virtual number, remember who else uses them. Many honest people rely on VoIP and virtual numbers every day:

• Small business owners who keep work and personal lines separate
• Remote workers using a company phone app
• Travelers and expats who keep a home country number online
• Privacy minded users who do not want to share their personal mobile number

A blanket ban on all VoIP numbers can block exactly the business customers you want most. The goal is to spot risky numbers, not to punish a technology.

What a VoIP Number Lookup Can Tell You

A good VoIP number lookup process looks at several clues together, not just one label. The useful questions are:

• Is the number valid and correctly formatted for its country?
• Who is the carrier, and is that carrier known for virtual or app based numbers?
• Is the number flagged as disposable or linked to spam?
• Is it already on your own blocklist from past abuse?
• Does the number's country match the user's IP location and other details?

The answers help you separate a stable business VoIP line from a throwaway number pulled from a public SMS site.

How Greip Phone Scoring Supports VoIP Screening

Greip's phone scoring, part of its data scoring and validation service, checks a number against the country you provide and returns clear, decision ready results. According to Greip's documentation, these include:

• Whether the number passes structure and validation checks for its country
• The carrier name, when carrier data is available
• Whether the number is flagged as disposable or spam
• Whether it matches your own blocklists or custom rules
• A low, medium, or high risk level, a suggested action of accept, review, or reject, and plain language reasons

Block, Challenge, or Allow: A Practical Signup Policy

Instead of one hard rule, sort phone numbers into three outcomes:

1. Block numbers that fail validation, are flagged as disposable, or match your blocklist, especially when other risk signals stack up.
2. Challenge numbers that look unusual but could be honest, such as a carrier known for virtual numbers on a new account. Ask for email verification, a payment method, or a short manual review before granting perks.
3. Allow valid numbers with a low risk level and no conflicting signals.

This approach stops the cheapest abuse while keeping signup smooth for real people.

Combine Phone Checks With Email and IP Signals

Phone data is strongest when it is not alone. Fraud rings often reuse patterns across several fields, so combining checks catches what any single check would miss:

• Phone plus email: a disposable number paired with a disposable email address is a strong warning sign
• Phone plus IP: a number from one country used from a VPN or hosting network in another deserves a closer look
• Phone plus velocity: many signups using numbers from the same carrier in a short window can point to a farm

Greip checks email addresses alongside phone numbers, and its VPN and proxy detection adds IP context. Together, they help you build a fuller picture as part of fake account prevention.

Common Mistakes When Screening Phone Numbers

Teams new to phone screening often make the same few mistakes:

• Relying on SMS codes alone: a code proves someone can receive a text, not that they are a real, unique customer
• Blocking all VoIP numbers: this hurts business users and travelers
• Ignoring country mismatches: a number, IP, and billing address from three different countries is worth a look
• Never updating blocklists: add numbers and patterns from confirmed abuse cases
• Checking only at signup: screen numbers again when users change them on an existing account

Test Numbers Before You Build Rules

You can see how phone screening behaves before you change your signup flow. Greip's free phone number validator checks whether a number is valid for a country and shows risk signals such as disposable flags, blocklist status, and a recommendation.

Try a handful of numbers from recent suspicious signups and a few from trusted customers. The difference will help you set fair thresholds for your own rules.

Frequently Asked Questions

What is a VoIP number lookup?

A VoIP number lookup checks a phone number for clues about whether it is a virtual or internet based number and how risky it is, such as its validity, carrier, disposable flags, and blocklist matches.

How can I tell if a phone number is VoIP?

Look at the carrier name, since some carriers mainly provide virtual or app based numbers, and check whether the number is flagged as disposable or spam. Combining these clues with IP and email data gives the clearest picture.

Why do fraudsters use virtual phone numbers?

Virtual numbers are cheap and quick to create, so fraudsters use them to pass SMS verification, run many accounts, and repeatedly claim promotions, free trials, or referral rewards.

Should I block all VoIP numbers at signup?

No. Many business users, remote workers, and travelers use VoIP numbers. Block numbers that are invalid, disposable, or on your blocklist, and add extra verification for others that look unusual.

Does Greip detect VoIP numbers?

Greip phone scoring returns validity, carrier name when available, disposable or spam flags, blocklist matches, a risk level, a recommendation, and reasons. It does not return a separate VoIP line type label.

Conclusion

Virtual phone numbers have made SMS verification far easier to fake, and attackers use them to bypass checks, create many accounts, and drain promotions. With so many registration attempts now coming from attackers, a phone number on its own is no longer proof of a real customer.

The answer is not to ban VoIP outright. Check validity, carrier, disposable flags, and your own blocklists, then block, challenge, or allow based on risk. Pair phone checks with email and IP signals for the strongest results. Start with Greip's data scoring and validation service to screen numbers before they turn into fraud.