What is FAS (False Answer Supervision) in VoIP and how to detect it

False Answer Supervision (FAS) is one of the most expensive problems a wholesale VoIP buyer can face, because it is designed to look like normal, successful traffic. A route affected by FAS can show a healthy answer rate while it quietly charges you for calls that never reached a real person. This guide explains what FAS is, how it typically shows up in your traffic, and the practical steps your NOC can take to detect it and keep it off your network.

What is False Answer Supervision?

In telephony, answer supervision is the signal that tells every network along the path that the called party has picked up. In SIP, that moment is marked by the 200 OK response to the INVITE. From that point on, the call is considered connected and billing starts.

False Answer Supervision happens when a carrier somewhere in the chain sends that answer signal even though the destination has not really answered. The caller may still hear ringing, a recorded message, silence or a looped audio clip, but every second after the false answer is billed. Because wholesale traffic is billed per minute, FAS turns unanswered or unreachable calls into revenue for whoever generates the false answer — and into cost for the buyer upstream.

Why FAS hurts wholesale carriers

• Direct cost: you pay your vendor for minutes that your own customer may dispute or refuse to pay.
• Misleading KPIs: FAS often pushes the answer-seizure ratio (ASR) up, so an affected route can look better than an honest one in a simple report.
• Customer complaints: end users hear ringing or strange audio while being charged, which damages trust in your service.
• Disputes and time: without evidence, it is hard to claim credits from a vendor, and investigations take NOC time away from other work.

Common forms of FAS

Early answer

The call is answered immediately, or very soon after the INVITE, before any real ringing could have happened. A tell-tale sign is an answer without any preceding 180 Ringing or 183 Session Progress with ringback.

Ringback after answer

The network sends 200 OK, but the caller keeps hearing a ringback tone. The audio says “still ringing” while the signalling says “connected”, so the ringing time is billed.

Recorded or repeated audio

Instead of a real person or the operator’s genuine announcement, the caller hears a recording, silence, or a short clip that is played on many different calls. Identical audio appearing on calls to different numbers is a strong signal.

Short-duration patterns

Some routes show many answered calls that all end after a similar, short duration. Real conversations have a natural spread of call lengths; artificial patterns tend to cluster.

How to detect FAS

No single check catches everything, so effective detection combines signalling, audio and statistics.

1. Analyse the SIP signalling of each call

Look at the sequence and timing of messages: was there a 180 or 183 before the 200 OK? How long after the INVITE did the answer arrive? A SIP ladder trace for suspicious calls makes these patterns visible at a glance and is useful evidence in a dispute.

2. Listen to, and analyse, the audio

Call recordings reveal ringback tones after answer, silence and recorded messages. Automated analysis can compare audio across calls and flag clips that repeat on calls to different destinations — something a human would rarely notice by spot-checking.

3. Watch route statistics

Compare each route’s ASR, ACD and call-duration distribution with other vendors for the same destination. A route with an unusually high answer rate combined with very short or very uniform call durations deserves a closer look. Sudden changes after a vendor update are also worth investigating.

4. Make controlled test calls

Calling numbers you control, or numbers whose behaviour you know, lets you compare what really happened at the destination with what the route reported. Keep test calls operator-controlled and limited in number so testing does not become a cost of its own.

5. Keep a list of suspect numbers

When individual calls are flagged, record the destination numbers involved. Repeated hits on the same numbers or ranges help you separate a systematic problem from a one-off.

How to avoid and respond to FAS

• Test before you scale. Put new routes through a test period with limited traffic before sending volume.
• Monitor continuously. FAS can appear on a route that was clean last week. Per-call and per-route verdicts that update automatically are far more effective than occasional manual checks.
• Collect evidence. Keep SIP traces, recordings and a clear reason for each verdict. Specific evidence makes vendor conversations faster and fairer.
• Agree on terms. Make sure your interconnect agreements describe how FAS disputes are raised and handled.
• Act on routing. Reduce or remove traffic from routes that keep showing FAS, and prefer vendors whose quality is consistently clean.
• Review regularly. Hourly or daily summaries help your team spot trends instead of reacting only to complaints.

Reading a FAS verdict

A practical way to communicate FAS findings is a simple three-level verdict per route:

• CLEAN – calls behave normally, with ringback before answer and natural conversation patterns.
• MIXED – some calls look suspicious; keep watching and spot-check recordings.
• FAS – clear signs such as early answer without ringback, recorded or repeated audio, or tell-tale short-duration patterns.

Attaching the reason to each verdict matters as much as the verdict itself: it tells the NOC what to check and gives the routing team something concrete to discuss with the vendor.

How VoxInspector helps

VoxInspector is our VoIP testing and monitoring platform for wholesale carriers on VOS3000 softswitches. It analyses live traffic per call and per route, gives CLEAN / MIXED / FAS verdicts with reasons, keeps a suspect number list with recording playback, and provides SIP ladder traces for investigation — alongside live ASR, ACD, PDD, packet loss and MOS monitoring.

Want to see how it works on your own traffic? Request a demo or learn more about VoxComms wholesale voice services.