FAS in VoIP: What It Is and Why It Matters

FAS (False Answer Supervision) is one of the most common issues in VoIP telephony, directly affecting billing accuracy, call quality, and customer trust. Below we explain what FAS is, why it occurs, and how it can be avoided.

What Does FAS Mean?

False Answer Supervision occurs when a call is marked as answered, even though the called party did not actually answer. From a SIP signaling perspective, the call is considered successful and billing starts, but no real conversation takes place.

In simple terms: the call is billed, but no one answered.

How False Answer Supervision Occurs

In real-world VoIP environments, FAS most commonly appears in the following scenarios:

• the call is routed to an operator announcement (“subscriber unavailable”);
• instead of ringback, the caller hears music, advertising, or a recorded message;
• the upstream provider sends a premature SIP 200 OK;
• GSM gateways or low-quality termination routes are used.

From the SIP protocol perspective, this looks like a valid Answer Supervision, but in reality it is false.

Why FAS Is a Serious Problem

False Answer Supervision leads to multiple negative consequences:

• customers are charged for unanswered calls;
• ASR and ACD statistics become distorted;
• real route quality is hidden behind misleading metrics;
• disputes with providers and partners increase.

FAS is especially critical for call centers, VoIP carriers, international termination, and prepaid services.

Where FAS Is Most Common

The highest risk of FAS is typically associated with:

• low-cost or “grey” international routes;
• GSM termination;
• VoIP ↔ PSTN interconnects;
• networks without proper early media control.

How to Avoid FAS

Effective FAS prevention requires continuous technical and operational monitoring:

• early media analysis (183 Session Progress with SDP);
• call filtering based on minimum ACD;
• voice, silence, and tone detection;
• ongoing CDR and route quality analysis.

FAS-Free Routes from SkyTel

SkyTel offers FAS-free CLI routes focused on real call quality rather than artificially inflated statistics.

We:

• use only verified routes with no false answer supervision;
• continuously monitor ASR, ACD, and actual call behavior;
• promptly eliminate problematic destinations and providers;
• ensure correct Answer Supervision and transparent billing.

Our approach fully eliminates situations where customers are charged for calls that were never answered.

Conclusion

FAS is not just a signaling issue — it is a real business risk. Choosing FAS-free routes and maintaining strict quality control are essential for fair billing, reliable KPIs, and long-term customer trust.

This is exactly the approach SkyTel follows in its VoIP services.