False answer supervision (FAS) is when a route signals that a call was answered although nobody picked up, so billing starts during ringing, on busy or unreachable numbers, or on a fake recording. You spot it by comparing the answer time in your CDRs with what really happened on test calls, and by looking for unanswered numbers that show billed minutes.
How FAS shows up
In SIP, the 200 OK response marks the moment a call is answered and billing begins. On a route with FAS, somewhere in the chain the 200 OK is sent too early or when no answer happened. The common forms are:
- Early answer: the call is marked answered while the phone is still ringing, so ringing time is billed.
- Answered failures: calls to busy, switched off or invalid numbers are marked answered and billed, often with silence or a tone played back.
- Fake ringback or recordings: the route plays a recorded ringback tone or an operator style announcement after the answer signal, making the caller wait while minutes run.
- Late release: the call is released several seconds after the other side hung up, adding billed time.
FAS is a form of billing fraud. It inflates minutes and ASR at the same time, so a FAS route can look like the best route on a simple dashboard.
Signs in your statistics
| Indicator | What FAS does to it |
|---|---|
| ASR | Unusually high compared with other routes on the same traffic |
| NER | Close to 100%, with very few busy or no answer causes |
| ACD | Lower than other routes, with many calls of a similar short length |
| Short calls | Many answered calls of 20 to 60 seconds with no conversation |
| PDD | Very short, because the answer comes almost immediately |
The definitions are in ASR, ACD, PDD and NER and NER vs ASR. None of these signs proves FAS on its own, but a route that beats every other route on ASR while losing on ACD deserves a test.
How to test for FAS
- Call numbers that will not answer: a handset left ringing, a busy line, a phone switched off, and a number that does not exist. None of these should produce a 200 OK.
- Time the answer: on calls you pick up, compare the moment you answered with the 200 OK timestamp in the SIP trace. A difference of more than about a second is suspicious.
- Listen: after the call is marked answered, is there a real person, or a ringback tone, silence or a message?
- Repeat at volume: some routes send FAS only on part of the traffic. Use a test-call platform or many numbers, not three test calls.
- Keep the evidence: SIP traces, recordings and CDRs, which you will need for a dispute.
Automated test call systems can detect FAS by analysing the audio after the answer signal, and some switches flag calls where voice activity starts long after the 200 OK. The full test routine is in how to test a VoIP route.
What to do if you find it
- Stop or reduce traffic on the route at once and tell the carrier, with examples. Your direct supplier may not be the source: FAS is often added further down the chain.
- Dispute the affected minutes. A good agreement says FAS calls are not payable and lets you withhold disputed amounts; see billing disputes and CDR reconciliation.
- Add explicit FAS clauses to new contracts: definition, right to block, credit for affected calls and a test procedure both sides accept, as listed in the interconnect agreement checklist.
- Monitor ACD and short calls per route daily, so a repeat shows up in hours instead of on the invoice.
VoIPMatcher stays in the middle of every route it sources and manages it with the supplier, so quality complaints go to one place. If you need VoIP termination to replace a route with billing problems, request a VoIP route.
Frequently asked questions
What does FAS mean in VoIP?
FAS stands for false answer supervision. A route with FAS sends the answer signal, the SIP 200 OK, when nobody has really picked up, so the call is billed during ringing, on busy or unreachable numbers, or while a recording plays. It inflates billed minutes and makes a bad route look good on ASR.
Is false answer supervision fraud?
Yes, it is treated as a form of billing fraud, because it charges for calls that were never answered. It is often added by a carrier further down the chain rather than by your direct supplier. Good interconnect agreements say FAS calls are not payable and let the buyer withhold or dispute the affected minutes.
Who pays for FAS minutes?
That depends on the contract. Wholesale agreements usually let the buyer dispute minutes proven to be false answers, with evidence such as SIP traces, recordings and CDRs. Without an explicit FAS clause, getting money back is harder, so add a definition, the right to block, credits and an agreed test method before you send volume.
What is early answer in VoIP?
Early answer is a FAS pattern where the route marks a call as answered while the phone is still ringing, so ringing time is billed. A related pattern is late release, where the call stays open and billed for several seconds after the other side hung up. Both show up when you compare SIP timestamps with what happened.
Which routes are most likely to have FAS?
FAS is most common on cheap routes with long carrier chains, where one hop can add false answers without the buyer seeing it, and on routes that end in grey termination. Routes priced far below the normal rate for a destination deserve a FAS test before volume, as do routes that suddenly beat every other route on ASR.
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