Your cheapest terminator shows a strong answer rate. Average call duration falls. Customers say they paid for ringing, for “out of coverage,” or for a dead line that still billed as answered.
That pattern points to False Answer Supervision (FAS): a hop in the path started the billing clock before a real conversation, or kept minutes running on fake audio.
This post is for wholesale and dialer operators who watch ASR and ACD per route. It covers how FAS works, how to spot it in CDRs, what to do with the supplier, and what MOR can flag on the call.
What FAS is
FAS is billing fraud on the answer event. A hop in the chain signals “answered” (or keeps the call up) so billable seconds run without a real talk path, or longer than the real talk path.
Common methods:
- Answer before the far end picks up, while you still hear ringback or silence.
- Play a recorded “subscriber unavailable” or voicemail clip and mark the call answered.
- Leave your leg up a few seconds after the far end hung up (late hang-up).
A few seconds on many calls is the business model. One CDR looks easy to ignore. The route invoice does not.
FAS is not a long post-dial delay. PDD is silence before ring feedback. FAS is money after a false or early answer.
Non-malicious cases exist: a VoIP-to-PSTN gateway that cannot map PSTN ring/connect states and forces CONNECT early. Treat that as a broken gateway. Move traffic until it is fixed.
Why your ASR and ACD lie
ASR and ACD are the numbers everyone quotes in wholesale chat. FAS breaks both:
| Metric | What FAS does |
|---|---|
| ASR | Inflates it. Fake answers count as answered. The route looks better than it is. |
| ACD | Pulls it down. Many fake answers last a few seconds. |
A route can show high ASR and low ACD at once. That combination is a FAS signal, not a quality win. EaseDial’s September 2026 route note makes the same point: short “answered” calls plus a suspicious ASR spike belong in investigation, not in the “good route” pile.
If you buy on ASR alone, you can reward the fraudster.
How it shows up on the call path
- Your subscriber or dialer places a call.
- Your softswitch routes by LCR or quality rules.
- Somewhere downstream, a hop answers early, injects audio, or holds the A-leg after B hangs up.
- Your CDRs and the supplier invoice show answered minutes the end user did not get.
You may never see the hop that did it. Your contracted provider can be clean and still land you on a dirty transit. Detection on your side still matters.
CDR fingerprints:
- Answered calls of about 1–10 seconds clustered on one terminator or destination.
- Caller hangs up on these short “answers”; no real conversation.
- Supplier billsec longer than your own connected time on the same calls.
- ASR jumps on one route with no change on your side.
Test that finishes the case: dial numbers you know are unassigned or out of service on that route. If they come back answered with billable time, the answer was fabricated.
Main FAS patterns
False billing without a real B-party. Fake ringback, looped audio, or a voicemail-style message. Invalid numbers that should fail with 4xx/5xx instead “connect” to audio.
Billing before B answers. A 200 OK (or equivalent billing start) while the far end is still ringing. Audio can stay ringback or a “hello, hello” clip.
Extra time after B disconnects. Late hang-up: B sends BYE; A stays up a few seconds, or an IVR plays after B is gone.
Fraudsters mix these into a small share of traffic, or turn them on only at night or above a channel count, so spot tests miss them. Production volume is when the pattern appears.
Example of a FAS fraud “business plan”:
What to do when you see it
- Segment CDRs by provider and destination. Compare short answered-call share and hangup direction across routes that carry similar traffic.
- Pull recordings on a sample of short “answered” calls if you store media. Looped ringback or a canned greeting on an “answered” call is evidence for the supplier ticket.
- Run known-bad numbers on the suspect route and on a premium control route.
- Open the dispute with CDRs, not with adjectives. Destination, timestamps, billsec vs your connect time, hangup causes. Pair with your partner invoice vs CDR workflow when the monthly file disagrees.
- Move traffic. Quality-weighted routing beats pure cheapest-route LCR when a cheap terminator buys ASR with fake answers. See The cheapest route that kills your ASR.
- Price the leak. If about 30% of minutes on a corridor show the pattern, that share is a premium you pay for nothing until the route is gone.
Stay off a dirty corridor when the rate sheet looks good. Do not wait for every end user to complain; most never name FAS.
What MOR can do on the call
MOR can stop a call when FAS control decides the provider is charging before the user has answered. Those calls end with hangup cause 248 (FAS control detected an issue). Use Last Calls and hangup reports to see which terminator and destination triggered it, then drop or demote that provider in LCR while you escalate.
Track ASR and ACD per provider and destination in the same reports you already use for route quality. A rising ASR with falling ACD on one terminator is the dashboard hint; 248 and short answered CDRs are the confirmation path.
Grey or “fake answer” routes are a known FAS environment. Prefer terminators you can test and replace. Fake ring on a device or provider can also mask delay; that is a separate setting and a separate conversation with the supplier.
MOR does not replace audio forensics or a full interconnect probe lab. It gives you a hangup you can act on and the metrics to spot the route before the invoice closes.
FAQ
Is every short answered call FAS?
No. Answering-machine detection, real quick hang-ups, and bad audio also create short answers. Compare against your other routes for the same destination and traffic type.
Can my direct provider be innocent?
Yes. Transit in the middle can introduce FAS. You still have to move traffic and send evidence upstream.
Is FAS the same as IRSF or balance drain?
No. IRSF and account takeover attack different money paths. FAS attacks the answer clock on termination routes. See also How VoIP providers get drained in 30 minutes for prepaid drain patterns.
Does a high ASR always mean a good route?
No. Check what the ACD and short-call share are doing on the same slice of CDRs.
Closing
FAS pays the fraudster in seconds and costs you in disputes, churn, and bad route decisions. Watch ASR and ACD together, hunt short answered clusters, test dead numbers, and cut the terminator when the evidence is clear.
If hangup 248 appears on a provider you care about, treat it as a route problem first, then a commercial conversation with CDRs in hand.
Seeing high ASR and short answered calls on one route?
Walk hangup cause 248 and Last Calls on the MOR demo, or talk through FAS route checks with us.