Fraudulent automated calls represent a growing threat to businesses and individuals across Europe. With 9 fraudulent calls per user per month according to NICE studies, three times the US rate, France has become the European country most targeted by robocalls. Understanding how robocall detection works and deploying appropriate technologies is essential for any IT director or telecommunications professional.
What is a Robocall and Why Detect Them?
Definition: Automated Call vs Phone Spam
A robocall is a phone call initiated by an automated system that delivers a pre-recorded or synthesized voice message. Unlike traditional phone spam where a human operator is present, robocalls are fully automated, allowing fraudsters to make thousands of simultaneous calls at minimal cost.
This distinction matters because detection technologies differ: detecting an automated call requires analyzing specific behavioral patterns that human calls don't exhibit.
France: Most Targeted European Country (9 calls/user/month)
The statistics are alarming: France records an average of 9 fraudulent calls per user each month, a figure three times higher than the US rate (approximately 3 calls/month). Several factors explain this situation:
- Operator fragmentation: the multiplicity of telecom players complicates implementing unified standards
- Regulatory delay: Europe hasn't yet adopted an equivalent to the US TRACED Act
- Lucrative market: French purchasing power attracts international fraudsters
- Exploitable VoIP infrastructures: the transition to all-IP facilitates spoofing
To learn more about phone identity theft, check our article on the STIR/SHAKEN protocol against spoofing.
Economic and Security Impact of Robocalls
The impact of robocalls extends far beyond mere nuisance. For businesses, consequences are multiple:
- Direct financial losses: CEO fraud, vishing, and tech support scams cost millions annually
- Reduced productivity: employees lose an average of 15 minutes daily filtering unwanted calls
- Reputation damage: when a company's number is spoofed, its credibility suffers
- Security risks: robocalls often serve as the initial vector for more sophisticated attacks
TDoS (Telephony Denial of Service) attacks represent a particular threat: by flooding a call center or emergency service with thousands of automated calls, attackers can completely paralyze an organization.
Robocall Detection Technologies
What technologies enable blocking automated calls? Several complementary approaches exist, each with its strengths and limitations.
STIR/SHAKEN: Authentication at the Source
STIR/SHAKEN (Secure Telephone Identity Revisited / Signature-based Handling of Asserted information using toKENs) is the most promising call authentication standard. This protocol enables:
- Cryptographically signing each call at origin with a digital certificate
- Verifying authenticity of the calling number at destination
- Assigning an attestation level (A, B, or C) according to the degree of trust
According to TransNexus, a leader in robocall mitigation solutions, STIR/SHAKEN provides complete traceability of the call chain. Companies wishing to deploy this technology can consult our guide to implement STIR/SHAKEN in your infrastructure.
SIP Analytics: Real-Time Behavioral Analysis
SIP Analytics constitutes an essential protection layer for automated call detection. This technology analyzes SIP flow characteristics in real time:
- Call volume by source: detection of abnormal peaks over a 60-minute sliding window
- Average call duration: robocalls typically have very short or very uniform durations
- Answer rate: suspicious hangup patterns
- IP geolocation: inconsistencies between displayed number and actual origin
Modern SIP Analytics systems are self-learning: they dynamically adjust their thresholds based on each source's history, avoiding false positives on legitimate high-volume callers.
Machine Learning and Reputation Scoring
How does machine learning detect fraudulent calls? Machine learning algorithms analyze dozens of parameters to calculate a reputation score from 0 to 100:
- Number history: previous complaints, community reports
- Temporal patterns: call time, day of week, frequency
- SIP metadata: header consistency, user-agent, codecs used
- Cross-network correlations: number behavior observed at other operators
A high score (close to 100) indicates a high-risk number. These systems enable proactive detection: a new fraudulent number can be identified in just a few calls, even before being manually reported.
Phone CAPTCHA: Human Verification
The phone CAPTCHA represents the last line of defense against robocalls. When a call is deemed suspicious by other detection methods, it's redirected to a CAPTCHA gateway that:
- Asks the caller to enter a random two-digit code
- Verifies the voice or DTMF response
- Authorizes the call if the answer is correct
- Blocks or reports if no valid response is provided
Autodialers cannot correctly respond to these tests, immediately identifying them as automated calls.
Mitigation vs Prevention: Two Complementary Approaches
What's the difference between robocall mitigation and prevention? These two terms, often confused, designate distinct strategies in fighting automated calls.
Robocall Mitigation: Blocking at Origin
Robocall mitigation refers to processes aimed at preventing fraudulent calls from leaving the originating network. It's a proactive approach where the originating operator:
- Analyzes all outgoing calls from its customers
- Blocks suspicious calls before they reach the public network
- Holds subscribers accountable in case of abuse
In the United States, the TRACED Act requires all operators to deploy a robocall mitigation program and register in the FCC database. Calls from uncertified operators must be blocked.
Robocall Prevention: Protecting at Destination
Robocall prevention protects subscribers by filtering incoming calls. This defensive approach allows:
- Analyzing each incoming call according to reputation criteria
- Blocking, redirecting, or marking suspicious calls
- Offering end users reporting tools
Prevention is more common because it doesn't require coordination with other operators, but it intervenes later in the call chain.
Why Combine Both Strategies
Optimal protection requires combining mitigation and prevention. Mitigation alone isn't enough because fraudsters can exploit unregulated foreign operators. Prevention alone lets fraudulent calls circulate on the network before being blocked, consuming resources.
The most advanced companies deploy both approaches simultaneously. Learn more about phone spam detection mechanisms to go deeper.
Advanced Detection Methods
Blacklists and Do Not Originate Lists
Blacklists allow blocking calls based on specific attributes:
- Numbers or prefixes identified as fraudulent
- IP addresses associated with known robocallers
- SPIDs/OCNs not authorized to use certain numbers
- Countries of origin suspicious for certain number types
Do Not Originate (DNO) lists are particularly effective: they catalog numbers that should never originate calls (service numbers, deactivated numbers). Any call claiming to come from a DNO number is automatically fraudulent.
Call Pattern Analysis (Volume, Duration, Geolocation)
Behavioral analysis examines call patterns across multiple dimensions:
- Instantaneous volume: a residential subscriber suddenly making 500 calls/hour is suspicious
- Call duration: robocalls often have very short durations (< 5 seconds) or abnormally uniform ones
- Call/answer ratio: robocall campaigns have very low answer rates
- Geolocation: a French number calling from an Asian IP is suspicious
Number Reputation Verification
Phone reputation services aggregate data from millions of users to calculate a trust score. These services query:
- Community reports
- Complaints filed with regulators
- Number assignment history
- Behavior patterns observed on the network
To verify a number's reputation, you can use our phone verification API which integrates these different sources.
TDoS Attack Detection
TDoS (Telephony Denial of Service) attacks use massive robocalls to overwhelm a target. Detection relies on:
- Traffic spike monitoring toward a single destination
- Source analysis: TDoS attacks typically come from multiple simultaneous origins
- Temporal correlation: suspicious call synchronization
- Automatic blocking with notification when a threshold is reached
Deploying a Detection Solution in France
The French MAN and Regulatory Requirements
Why does France receive so many automated calls? Partly because the European regulatory framework remains less constraining than the US TRACED Act. However, several mechanisms exist:
- Bloctel: the telemarketing opt-out registry, though it doesn't specifically target fraudulent robocalls
- ARCEP numbering plan: framework for number allocation and use
- MAN (Number Authentication Mechanism): French initiative to authenticate caller identity
ARCEP is actively working on strengthening operator obligations regarding spoofing and fraudulent call prevention.
Integration with Existing SIP Systems
Deploying a robocall detection solution in existing infrastructure requires:
- Current architecture analysis: optimal insertion points for filtering
- Integration mode selection: SIP proxy, SBC, or passive flow analysis
- Rule configuration: thresholds, actions (blocking, marking, CAPTCHA)
- Pilot phase: testing on a restricted scope before generalization
Modern solutions like TransNexus's ClearIP integrate with most market SIP equipment (Ribbon, Metaswitch, etc.) and natively support STIR/SHAKEN.
Measuring Effectiveness: KPIs and Reporting
Tracking detection solution effectiveness relies on key indicators:
- Detection rate: percentage of robocalls identified out of total received
- False positive rate: legitimate calls blocked by mistake (should remain < 0.1%)
- Detection time: delay between a new campaign's appearance and its detection
- User satisfaction: reduction in unwanted call complaints
Regular reporting allows fine-tuning rules and demonstrating solution ROI to management.
FAQ
What's the difference between a robocall and a spam call?
A robocall is a fully automated call using a pre-recorded or synthesized message. A spam call can be made by a human (unsolicited telemarketing). Not all robocalls are spam (some are legitimate), and not all spam is automated.
Is STIR/SHAKEN mandatory in France?
No, not yet. Unlike the United States where the TRACED Act mandates it, France and Europe haven't made STIR/SHAKEN mandatory. However, ARCEP encourages its adoption and the French MAN is largely inspired by it. Companies can implement it voluntarily for better protection.
How do I know if a call is authenticated?
When a call is authenticated via STIR/SHAKEN, attestation information (level A, B, or C) is transmitted in SIP headers. Some phones and applications display a visual indicator (checkmark, "Verified" mention). Your operator or telephony solution can also expose this information.
Do anti-robocall solutions block legitimate calls?
The risk of false positives exists but remains low with modern solutions (< 0.1%). Self-learning systems adjust their thresholds based on each caller's history. Suspicious calls can be redirected to a CAPTCHA rather than blocked directly, allowing legitimate callers to prove their humanity.
Sources: TransNexus - Robocall Mitigation, ARCEP - News, Bloctel
