Every verdict CallerSift returns leans on a deceptively simple question: what kind of line is this number? Not who owns it, not what they want, just what the line physically and commercially is. It turns out that one attribute separates real callers from manufactured ones better than anything else you can learn in half a second.
Here is the field guide.
Mobile
A number provisioned by a wireless carrier for a handset. The overwhelming majority of genuine consumer calls in pay-per-call arrive from mobile lines, for the obvious reason: real people call from the phone in their hand.
Mobile numbers are attached to carrier accounts, which cost money and require some form of identity. That friction is exactly what fraud avoids, so mobile lines carry the strongest presumption of a human on the other end.
Landline
A traditional wireline number tied to a physical address. Rarer every year, but still a meaningful slice of traffic in older demographics and rural markets, and about as hard to fake as a street address. A landline caller asking about final expense insurance is about the most organic thing that can happen to a campaign.
Fixed VoIP
A VoIP line anchored to a registered business or residential account: think office phone systems, cable-bundle home phones, hosted PBXs. The number lives in software, but there is an account, an address, and a paying customer behind it.
Fixed VoIP is mostly legitimate. It is how businesses call you. In consumer campaigns it shows up less often, and context matters more, which is why a verdict weighs it rather than sentencing it.
Non-fixed VoIP
A number that exists purely in software with no physical anchor and, typically, no verified identity. This is the native habitat of burner apps: TextNow, Google Voice, Talkatone, and every disposable-number service you have never heard of.
Non-fixed VoIP is not automatically a criminal. It is, however, the line type of choice for incentive fraud, dialer farms, and promo abuse, because numbers cost nothing and rotate freely. In pay-per-call screening it is the highest-risk classification by a wide margin, and it is the reason line type screening works at all: a burner can change its number all day, but it cannot change what the number is.
Toll-free
An 8xx number calling you inbound is an oddity. Consumers do not dial out from toll-free lines; call centers, resellers, and spoofing setups do. Real prospect traffic from toll-free origins rounds to zero, which makes it an easy read.
Where the false positives hide
Line type is strong signal, not gospel, and honest screening admits it:
- Some real people run their lives on Google Voice numbers, especially the privacy-conscious ones.
- Small businesses call from fixed VoIP constantly, which matters if your campaign wants business callers.
- Number porting means a line's history can straddle categories; carrier intelligence reflects the current classification, which is the one that matters.
This is why CallerSift shows you the reason behind every verdict in the lookups table, and why the verdict logic treats line type as the anchor of a decision rather than the whole decision. When a blocked caller turns out to be your one legitimate Google Voice customer, you can see exactly why it happened instead of arguing with a black box.
Reading your own mix
Pull up your lookups view and look at the line-type spread per source. Clean consumer traffic is unmistakable: a tall mobile bar, a modest landline bar, a sliver of everything else. A source shipping you 25 percent non-fixed VoIP is not reaching a different audience, it is manufacturing one.
That is the whole trick. You cannot interview every caller in half a second, but you can know what kind of line they chose to call you from, and that choice is rarely an accident. Run your traffic through it and see your own spread.