How to Identify and Block Fake Phone Numbers in SaaS Applications

Many SaaS applications use phone numbers to confirm the identity of new users. This process helps keep accounts secure and reduces the chances of automated bots or fake signups.

However, not every phone number entered during signup belongs to a real person. Some numbers have been created for temporary use or to hide a user's true identity.

Understanding the kinds of phone numbers that show up in SaaS applications can help teams distinguish between genuine users and those trying to bypass verification. This section explains what makes a phone number disposable, temporary, or virtual, and why these types of numbers create risks for software platforms.

What Is a Disposable Phone Number and Why Does It Matter

A disposable phone number works like a temporary email address—you can get one quickly, use it for verification, then throw it away. These numbers go by several names: temporary phone numbers, virtual phone numbers, or throwaway numbers.

Here's how they differ from regular phone numbers:

• Regular mobile numbers: Issued by major carriers like Verizon or AT&T, tied to a specific person, and cost money each month
• Disposable numbers: Created through apps or websites, available instantly, and often free or very cheap
• VoIP numbers: Use internet connections instead of cell towers, making them easy to create in bulk

Most disposable phone number services use Voice over Internet Protocol (VoIP) technology. This means the numbers aren't connected to physical SIM cards or specific devices. Instead, they route calls and texts through internet servers.

The problem for SaaS platforms is simple: when someone uses a disposable number to sign up, you can't reach them later. These numbers often stop working within hours or days, leaving you with dead contact information in your database.

Business Risks of Disposable and VoIP Numbers in SaaS

Fake phone numbers create several specific problems for SaaS companies. The most expensive issue is SMS pumping, where attackers use premium-rate disposable numbers to drain your SMS budget. When your application sends verification codes to these numbers, you get charged premium rates—sometimes $1 or more per message.

Trial abuse happens when users exploit your free trial offers by signing up repeatedly with new disposable numbers. Since each number appears legitimate to your system, the same person can create dozens of accounts and extend their free usage indefinitely.

Your customer data quality drops significantly when disposable numbers fill your database. Marketing campaigns fail because messages bounce back, customer support can't reach users who need help, and your conversion tracking becomes unreliable.

Here are the main risks:

• SMS pumping attacks: Premium-rate numbers can cost $0.50-$2.00 per verification message
• Trial exploitation: Users create multiple accounts to bypass usage limits
• Support inefficiency: Teams waste time trying to contact unreachable users
• Marketing waste: Email and SMS campaigns show poor performance due to invalid contacts

Core Signals That Identify Fake Phone Numbers in Real Time

Phone validation APIs analyze multiple data points to spot suspicious numbers instantly. These systems check technical details that reveal whether a number belongs to a real person or comes from a disposable service.

Carrier Type and Line Status

Every phone number connects to a specific type of network. Mobile carriers like T-Mobile issue numbers for cell phones, while VoIP providers create numbers that work over the internet. Phone number intelligence systems can identify which category a number falls into.

Line status tells you if a number can actually receive calls and texts right now. Active lines work normally, while inactive lines have been disconnected or suspended. Disposable services often recycle numbers quickly, so a number might be active when someone signs up but inactive days later.

HLR and Porting Information

The Home Location Register (HLR) is a database that mobile networks use to track their subscribers. An HLR lookup reveals if a number is currently active, which carrier owns it, and whether the phone is roaming in another country.

Number porting happens when someone moves their phone number from one carrier to another. Frequent porting—especially to obscure VoIP providers—can signal that a number is being used for suspicious purposes.

Prefix and Range Analysis

Phone numbers follow predictable patterns. Each country assigns specific number ranges to different types of services. For example, certain prefixes in the US are reserved for VoIP providers, while others belong to major mobile carriers.

Disposable phone number services often use the same ranges repeatedly. By checking these patterns, validation systems can flag numbers that come from known temporary providers.

Digital Footprint and Activity Checks

Real phone numbers leave digital traces over time. They get linked to social media accounts, used for online purchases, and connected to various services. This creates a digital footprint that proves the number belongs to an actual person.

Disposable numbers have little or no history. They rarely appear in public databases, aren't linked to established accounts, and show no signs of regular use. This absence of activity is itself a strong signal.

Step-by-Step Process to Block Suspicious Numbers at Signup

Blocking fake phone numbers requires a systematic approach that catches problems early without frustrating legitimate users. Here's how to implement this process:

Step 1: Validate Syntax and Format

Check that the phone number follows international formatting standards before sending it to your validation API. This means verifying the country code, area code, and total digit count match what's expected for that region.

Basic format validation catches obvious errors like missing digits or invalid country codes. This step prevents wasted API calls and filters out clearly fake entries.

Step 2: Perform Carrier and Line-Type Lookup

Send the formatted number to a phone validation API that returns carrier information and line type. The API will tell you if the number belongs to a major mobile carrier, a VoIP provider, or a known disposable service.

This lookup also reveals whether the line is currently active and can receive SMS messages. Numbers that show as inactive or unreachable get flagged for further review.

Step 3: Apply Risk Scoring Thresholds

Most phone validation services return a risk score between 0 and 100. Lower scores indicate trustworthy numbers, while higher scores suggest potential problems. Set thresholds based on your risk tolerance:

• Low risk (0-30): Allow registration automatically
• Medium risk (31-70): Require additional verification
• High risk (71-100): Block registration or trigger manual review

Step 4: Trigger Fallback Verification for Borderline Scores

Numbers in the medium-risk range might be legitimate users on VoIP services or legitimate but unusual carriers. Instead of blocking them outright, offer alternative verification methods like email confirmation or authenticator apps.

Step 5: Log and Monitor Outcomes for ML Feedback

Track which numbers you block, allow, or flag for review. This data helps you adjust your thresholds over time and train machine learning models to improve detection accuracy.

How to Integrate a Phone Validation API Without Slowing UX

Adding phone number validation to your registration flow doesn't have to slow down the user experience. The key is running validation checks at the right time and handling delays gracefully.

Asynchronous Pre-Send Checks

Validate phone numbers before sending OTP codes, not after. When a user enters their number, send it to your validation API in the background while they're still filling out other form fields. If the number passes validation, send the OTP. If it fails, show an error message without wasting money on SMS delivery.

Caching and Retry Strategies

Cache validation results for recently checked numbers to avoid duplicate API calls. If someone tries to register with the same number twice in a short period, use the cached result instead of making a new request.

Implement retry logic for API timeouts or temporary failures. Try the validation call once or twice, then fall back to allowing the registration if the service is unavailable.

Handling API Timeouts Gracefully

When validation APIs are slow or unresponsive, don't make users wait indefinitely. Set reasonable timeouts (2-3 seconds) and have a fallback plan. You might temporarily skip validation or show a message explaining the delay while processing continues in the background.

Key Metrics to Monitor After Deployment

Track these specific metrics to measure how well your phone validation system is working:

OTP Delivery Success Rate

Measure what percentage of verification codes actually reach users' phones. A high success rate (above 95%) indicates that you're filtering out unreachable numbers effectively while still serving legitimate users.

Registration Completion Rate

Compare how many users complete registration before and after implementing validation. A significant drop might mean your validation is too strict and blocking real users.

Fraudulent Account Decline

Count how many fake accounts, chargebacks, and abuse incidents you prevent. This metric shows the direct security benefit of your validation system.

SMS Spend per Verified User

Calculate your SMS costs divided by the number of successfully verified users. This number should decrease as you filter out premium-rate and invalid numbers.

Choosing a Vendor for Coverage Latency Price and Uptime

When selecting a phone validation provider, evaluate these technical criteria:

Country and Carrier Coverage

Check that your vendor covers all the regions where your users sign up. Global coverage matters for international SaaS applications, while local carrier relationships improve accuracy for specific countries.

Average and P99 Latency Benchmarks

API response times directly impact user experience. Look for average response times under 200ms and P99 latency (slowest 1% of requests) under 500ms.

SLA and High Availability

Review service level agreements for uptime guarantees. Mission-critical applications need 99.9% uptime or better, with clear escalation procedures when issues occur.

Transparent Pricing Models

Compare per-request pricing, volume discounts, and any hidden fees. Some providers charge extra for premium features or support, which can significantly impact total costs.

Next Steps to Secure Your Platform With NumCheckr

NumCheckr provides real-time phone number validation with response times under 80 milliseconds. The service covers over 120 countries and uses HLR lookup, carrier verification, and risk scoring to identify disposable and virtual phone numbers.

The API integrates directly into registration flows and returns detailed information about each number, including carrier type, line status, and risk assessment. This helps SaaS platforms block suspicious numbers while maintaining a smooth experience for legitimate users.

Start protecting your platform by creating a free trial account at numcheckr.com/register.

FAQs About Detecting and Blocking Disposable Phone Numbers

❓ How can I distinguish legitimate VoIP business numbers from disposable ones?

Business VoIP numbers typically come from established providers like RingCentral or 8x8 and maintain consistent carrier information over time. Disposable VoIP numbers frequently change carriers, show up on known temporary provider lists, and lack any digital history or business registration records.

❓ What backup verification methods work when phone validation blocks a number?

Email verification links, authenticator app setup (like Google Authenticator), or manual account review by your support team provide alternative ways for legitimate users to verify their identity when their phone number cannot receive SMS codes.

❓ How frequently do disposable phone number ranges change?

New disposable number providers launch weekly, and existing providers regularly acquire new number ranges. Real-time validation APIs automatically detect these changes, while static blocklists need weekly updates to stay current.

❓ Can phone validation be combined with other fraud detection methods?

Phone number validation works well alongside device fingerprinting, IP geolocation analysis, and behavioral monitoring. Combining multiple signals provides more accurate fraud detection than relying on phone validation alone.