How Phone Spam Detection Works in 2025

Every day, millions of unwanted calls are blocked before they even ring your phone. But how do these systems know a number is spam? Let's dive into the mechanics of phone spam detection.

The 3 Pillars of Spam Detection

Phone spam detection relies on three complementary mechanisms working together to protect users.

1. User Reports

This is the foundation of the entire system. When a user reports a number as spam on an app like Truecaller or Hiya, this information is shared with the entire community.

The principle is simple: if 100 people report the same number in 24 hours, there's a high chance it's actually spam. These apps have hundreds of millions of users worldwide, creating a massive collaborative database.

According to BusinessWire, Truecaller surpassed 400 million active users in 2024, and has blocked over 37.8 billion unwanted calls globally.

2. Behavioral Analysis

Detection algorithms analyze a number's calling behavior:

• Call volume: a number making 500 calls per day has a suspicious profile
• Call duration: many short calls (< 10 seconds) = telemarketing signature
• Answer rate: if nobody picks up, it's a bad sign
• Timing: calls outside legal hours (before 10am, after 8pm)
• Geographic spread: calling all over France in a few hours is suspicious

3. Machine Learning

Modern systems use artificial intelligence to detect patterns invisible to the human eye. Machine learning models are trained on millions of calls to recognize:

• Number sequences used by call centers (e.g., +33 6 XX XX XX 01, 02, 03...)
• Abnormal activity spikes
• Correlations between different indicators

These algorithms constantly improve through feedback loops: each report refines the model.

How French Carriers Detect Spam

Orange: Built-in Anti-spam

Orange offers an Anti-spam calls feature integrated into the "Orange et moi" app. The system analyzes incoming calls and displays a "Spam likely" warning when a number is suspicious.

Orange relies on:

• Its own database of reported numbers
• Network data (call volume per number)
• Partnerships with specialized providers

SFR, Free and Bouygues

Other carriers also offer solutions, often through:

• Built-in or optional filtering options
• Integration of third-party anti-spam technologies
• Dedicated apps (SFR Security, etc.)

Truecaller vs Hiya: Anti-spam Giants

Truecaller

Truecaller has 400 million active users worldwide. The app identifies unknown callers and automatically blocks known spam. Its strength: the size of its community that feeds its database in real-time.

Hiya

Hiya positions itself more in B2B. The company provides its technology to telecom operators and smartphone manufacturers (Samsung, notably). If your phone displays "Spam likely" without a third-party app, Hiya is likely behind it.

How a Number Becomes "Spam"

Here's the typical journey of a number that ends up blacklisted:

1. Day 1-3: The number starts making many calls
2. Day 3-7: First reports arrive on Truecaller/Hiya
3. Day 7-14: Report volume reaches a critical threshold
4. Day 14+: The number is marked "spam" and automatically blocked

This delay can be much shorter (a few hours) if the number has very aggressive behavior or uses already-monitored prefixes.

For more on timing, see our guide: How long does a number stay blacklisted?

Detection Limitations

False Positives

The main issue: legitimate numbers can be marked as spam. This happens when:

• A business makes many calls (even legitimate ones)
• Unhappy customers report abusively
• The number was recycled (former spam)

That's why it's crucial to monitor your numbers' reputation if you do telephone prospecting.

Evasion Techniques

Spammers constantly adapt:

• Number rotation: changing numbers before being detected
• Spoofing: impersonating legitimate numbers
• International calls: bypassing French rules

It's a permanent cat-and-mouse game between spammers and detection systems.

How to Protect Your Business Numbers

If you do legitimate telephone prospecting, here's how to avoid being marked as spam:

1. Respect volumes: don't exceed a certain number of calls per day
2. Respect hours: 10am-1pm and 2pm-8pm only
3. Vary your numbers: use a sufficient number pool
4. Let them rest: apply rest time between campaigns
5. Monitor continuously: use a verification service to detect problems early

FAQ

Can I know if my number is marked as spam?

Yes, you can use verification services like HUHU to know your numbers' status across different carriers and apps.

How do I remove my number from a spam list?

It's possible but not automatic. See our guide: How to unblock a spam number.

Do algorithms make mistakes often?

Yes, false positives exist. That's why systems typically use multiple sources before definitively marking a number as spam.

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Sources: Truecaller, Hiya, BusinessWire