SimPro — Extend Your SIMs Life

Call Filter is an intelligent call screening and filtering solution designed to protect GSM termination operations from the single biggest threat: test calls from anti-fraud companies. Mobile operators partner with specialized anti-fraud firms (Amdocs, ARRAXXE, BICS, CSG, FRAUDBUSTER, LATRO, Mobius, Revector, SIGOS, XINTEC, ZIRA) that inject test calls into the VoIP wholesale ecosystem. When these calls reach your GSM gateway SIM cards, the SIMs are flagged and blocked — often within hours.

Call Filter sits between your VoIP traffic sources and your GSM termination infrastructure, analyzing every call in real-time before it reaches your SIM cards. Using advanced whitelist/blacklist management, A+B number verification, IVR DTMF authentication, HLR lookups, and dynamic blacklists, Call Filter blocks suspicious calls while allowing legitimate traffic to pass through seamlessly. The result: dramatically extended SIM lifetimes, improved ASR/ACD metrics, and significantly higher termination revenue.

How Anti-Fraud Test Calls Work

Anti-fraud companies deploy sophisticated networks to detect and block GSM termination operations. Their primary weapon is the test call — artificially generated calls designed to expose termination SIM cards. Here's how the attack works:

• Injection Points: Test calls are injected through multiple channels: VoIP retailers, calling card services, wholesale carriers, and even direct SIP accounts. Anti-fraud companies build extensive networks to ensure their test calls reach every possible route.
• Known Test Numbers: Anti-fraud companies maintain databases of test numbers. When these numbers are dialed through your termination route, they immediately know a SIM card is being used for termination.
• White List Poisoning: Advanced anti-fraud systems build their own whitelists from both legitimate traffic and operator CDRs. They then use these "poisoned" whitelists to send test calls that appear to come from trusted sources.
• Real-Time Monitoring: When a test call reaches your SIM and connects, the anti-fraud system records the SIM's IMSI, IMEI, and BTS location. This data is shared across operator networks, leading to rapid blocking.

The Devastating Impact on Your Business

Test calls cause cascading damage that extends far beyond a single blocked SIM:

• Rapid SIM Blocking: A single test call can trigger SIM blocking within hours. In aggressive markets, unprotected SIMs may last only 24-72 hours.
• Mass Blocking Events: When anti-fraud systems detect one blocked SIM, they often analyze the entire gateway, leading to mass blocking of all SIMs sharing the same IMEI or BTS location.
• Revenue Destruction: Each blocked SIM represents lost top-up credit, wasted acquisition cost, and days of lost termination revenue while replacements are sourced and activated.
• Route Instability: Frequent SIM blocking causes route downtime, which damages your reputation with wholesale clients. Modern softswitches automatically detect downtime and stop sending traffic to unreliable routes.
• ASR/ACD Degradation: Test calls that connect but have zero duration artificially deflate your ASR and ACD metrics, making your routes less attractive to buyers.

Stage 1: Pre-Call Analysis & Whitelist Verification

When an incoming VoIP call arrives at your infrastructure, Call Filter intercepts it before it reaches your GSM gateway. The first stage analyzes the call against your configured whitelist rules:

• A-Number Whitelist: Check if the caller ID exists in your trusted caller database. Calls from unknown or unverified caller IDs are flagged for further analysis or rejected.
• B-Number Whitelist: Verify the called number against your destination whitelist. Test calls often target specific numbers known to anti-fraud systems.
• A+B Combination Filter: The most powerful protection — verify that the specific caller-to-called number combination exists in your historical traffic database. Anti-fraud test calls use number pairs that have never appeared in legitimate traffic.
• Per-SIM Whitelists: Bind specific trusted numbers to individual SIM cards, ensuring each SIM only receives calls from its pre-approved contact list.

Calls that pass whitelist verification proceed to the next stage. Calls that fail are immediately rejected with a configurable SIP response code (typically 503 Service Unavailable).

Stage 2: Blacklist & Dynamic Threat Detection

Calls that pass whitelist screening undergo blacklist and dynamic threat analysis:

• Static Blacklist: Block known test numbers, spam sources, and previously identified anti-fraud injection points. Static blacklists are manually maintained and shared across the Call Filter user community.
• Dynamic Blacklists: Automatically detect and block numbers exhibiting suspicious patterns:
  • Repetitive calls to the same number with low ASR
  • Calls with abnormally short durations (<5 seconds)
  • Numbers calling at unusual frequencies or times
  • Numbers with CLI/CLD patterns matching known test call signatures
• Prefix & Suffix Filtering: Block entire number ranges associated with test call services, premium numbers, or known fraud sources.
• Originator Analysis: Track ASR and ACD per traffic source. If a specific originator shows suspicious metrics, their traffic can be quarantined for manual review.

Stage 3: IVR Authentication & HLR Verification

For calls that pass both whitelist and blacklist screening, Call Filter offers additional verification layers:

• IVR DTMF Challenge: The caller hears a prompt like "Press 1 to continue your call." Real human callers can respond; automated test systems and robocalls cannot. Failed IVR challenges result in immediate call rejection.
• HLR Lookup: Query the Home Location Register to verify the caller's number is active, assigned to a real mobile subscriber, and not flagged as suspicious. HLR lookups can detect virtual numbers, disconnected lines, and test call services.
• Messenger Verification: Check if the caller's number has an active WhatsApp, Viber, or Telegram account. Legitimate mobile numbers typically have messaging accounts; test call systems and virtual numbers usually do not.
• Two-Stage Dialing: For high-risk destinations, implement two-stage dialing where the caller must enter a PIN or destination number, adding another barrier for automated test systems.

Stage 4: Intelligent Routing & Post-Call Learning

After a call is approved and connected, Call Filter continues learning and optimizing:

• CDR Analysis: After call completion, analyze duration, answer status, and quality metrics. Calls with zero duration or immediate hangups are flagged for blacklist review.
• Auto-Whitelist Building: Successful calls with good ACD automatically add the A+B number pair to the whitelist, expanding your trusted database over time.
• Pattern Learning: The system learns normal traffic patterns per originator, destination, and time-of-day. Deviations trigger alerts or automatic filtering adjustments.
• Feedback Loop: When SIMs are blocked, analyze which calls preceded the blocking event to identify new test call signatures and update filtering rules proactively.
• Community Intelligence: Benefit from shared threat intelligence across the Call Filter user network, receiving automatic updates about new test call numbers and anti-fraud tactics.

GSM termination markets in Africa, the Middle East, and Southeast Asia face the most aggressive anti-fraud systems and highest test call injection rates. Call Filter is specifically calibrated for these challenging environments with pre-configured profiles for major operators in Nigeria, Ghana, Kenya, Uganda, Tanzania, South Africa, Ethiopia, Cameroon, Pakistan, Bangladesh, and other high-volume termination destinations.

In these markets, unprotected SIM cards may be blocked within 24-72 hours. Call Filter users typically achieve 5-10x longer SIM lifetimes by eliminating the test calls that trigger blocking. The system is continuously updated with new threat intelligence specific to each market, ensuring your filtering rules stay ahead of evolving anti-fraud tactics.

High-Risk Market Challenges

• Extremely aggressive anti-fraud partnerships (Amdocs, Revector, SIGOS heavily active)
• High ratio of test calls mixed with legitimate traffic (up to 30% in some routes)
• Rapid SIM blocking within 24-72 hours for unprotected operations
• Mass blocking events affecting entire gateway farms
• Advanced white list poisoning by anti-fraud companies
• VoIP wholesale carriers unable to guarantee clean traffic

Call Filter Solutions

• Pre-configured anti-fraud profiles for 50+ operators worldwide
• Enhanced A+B filtering tuned for regional test call patterns
• Community threat intelligence sharing across user network
• Automatic rule updates when new test call signatures are detected
• Multi-layered filtering (whitelist + blacklist + IVR + HLR) for maximum protection
• Dedicated support team with deep expertise in regional anti-fraud tactics