Block spoofed inbound calls

A caller presents a number that is not theirs, often a local one or one that looks like a bank or a supplier. It is the mechanism behind most voice fraud.

A caller presents a number that is not theirs, often a local one or one that looks like a bank or a supplier. It is the mechanism behind most voice fraud, and it works because people trust what their phone shows them.

Blocking those calls runs on STIR/SHAKEN inbound verification, and CLASSOUND offers it for US numbers only. On a Dutch number it cannot be switched on.

How the verification works

Every telephone provider holds a digital certificate from an authority the other providers trust. When a call is placed, the originating provider states how sure it is that the caller may use the number it is presenting, and signs that statement. CLASSOUND checks the signature against the public certificate of that provider before the call reaches you.

The three levels of certainty

That statement comes in three grades, and knowing them explains why legitimate calls sometimes look doubtful.

This is the reason a real supplier can fail verification. A number behind their PBX gets B rather than A, not because anything is wrong but because their provider cannot confirm that last step. If a supplier says they cannot reach you, check this before assuming a fault at your end.

It does not switch itself on

Verification is not a toggle in a settings screen. It is a rule in the dialplan that reads the attestation the originating provider attached and decides what to do with the call, so somebody has to write that rule and decide what each level should mean for you.

That decision is the interesting part rather than the installation. Blocking everything below A stops a genuine supplier whose number sits behind their own PBX. Marking the call instead, so the agent sees it was not fully verified, keeps the information without losing the call. Tell us which of the two suits your desk and we will set it up with you.

What it will not catch

• Calls from a real number the caller genuinely controls.
• Calls from countries where verification is not in place.
• Anything where the deception is what is said rather than the number shown.

It reduces a category of attack rather than removing fraud. Staff still need to know that no bank asks for credentials by phone.

The other direction

To make your own outbound calls more trusted, present a consistent verifiable number and consider branded calling, so your name appears next to it.

See also

• Branded calls with Extended CNAM
• Configure caller ID