The short answer
Carriers no longer trust caller ID blindly. Every US call now carries a STIR/SHAKEN signature saying how sure the originating provider is that the caller may use that number, rated Level A, B, or C. Weak signatures and unfamiliar numbers get labeled "Spam Likely" or blocked. FreeCallMe's dialer sends the strongest signal, Level A attestation, and registers its numbers to build a trust score, which is everything a VoIP provider can control. The receiving carrier still makes the final call.
How caller ID works on a traditional call
On a landline or cell call, caller ID travels alongside the call through the carrier networks. Your carrier attaches your number, the receiving carrier displays it. The system is fast, but it was never designed with verification in mind. It simply trusts whatever number is attached.
That became a serious problem once VoIP made it trivial to attach any number to a call. A scammer could make a call look like it came from your bank, a government agency, or someone you know. This is caller ID spoofing, and it drove a wave of robocall fraud that taught people to stop answering unfamiliar numbers.
STIR/SHAKEN: the industry's answer
In response, the FCC mandated a framework called STIR/SHAKEN (Secure Telephone Identity Revisited, and Signature-based Handling of Asserted information using toKENs) for US carriers and VoIP providers. It has been required for the major carriers since 2021.
The core idea: instead of trusting the number attached to a call, every call gets a digital signature from the originating provider that asserts how confident they are that the caller is authorized to use that number. The signature travels with the call, and the receiving carrier checks it. It carries an attestation level of A, B, or C.
| Attestation | What the provider is asserting | Spam flag |
|---|---|---|
| Level A (full attestation) | The caller is a verified customer the provider knows is authorized to use this number. This is the gold standard. | Least likely to be flagged |
| Level B (partial attestation) | The call came from the provider's network, but the provider cannot confirm the caller is authorized to use that specific number. | Sometimes flagged |
| Level C (gateway attestation) | The provider received the call from another carrier and cannot verify anything about its origin. | Most likely to be flagged |
When a call arrives, the receiving carrier reads the attestation. Calls with Level C, or with no STIR/SHAKEN signature at all, are far more likely to be flagged as potential spam or blocked outright.
Why VoIP calls get labeled "Spam Likely"
Several factors push a VoIP call toward a spam label.
Low or missing attestation. A VoIP number that has not been registered with a proper STIR/SHAKEN-compliant provider carries no attestation or only Level C. Carriers treat that as a yellow or red flag before the phone even rings.
Unregistered numbers. Carriers lean on databases like the Free Caller Registry of known legitimate callers. A number that is not registered starts with no trust score and builds suspicion the moment its calling pattern looks unusual.
Call behavior patterns. The analytics platforms behind the labels (Hiya and First Orion power 'Spam Likely' on the major US networks) watch call volume, time-of-day patterns, and whether people answer, reject, or report calls. A new VoIP number that dials many different people quickly looks like a robocaller even when it is not.
Carrier-specific policies. T-Mobile, Verizon, and AT&T each set their own filtering aggressiveness, and T-Mobile has historically been the most aggressive. The same call can arrive clean on one network and 'Spam Likely' on another.
How FreeCallMe handles this
FreeCallMe's dialer sends STIR/SHAKEN Level A attestation, the strongest verification signal a US originating provider can attach, through its telephony carrier. In practice that means calls from the dialer arrive with the clearest possible "this caller is authorized to use this number" stamp for the receiving carrier to read.
FreeCallMe also registers its numbers so they build a baseline trust score in the reputation systems the major carriers use, such as the Free Caller Registry that feeds T-Mobile and Verizon Wireless.
None of this guarantees a call is never flagged. Individual carriers and their analytics partners make their own independent decisions, including on behavior. But it is everything within a VoIP provider's control to maximize the odds your call goes through cleanly.
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Why calls to India and other countries are harder to connect
International compliance with STIR/SHAKEN is uneven. The framework was built for the US and Canada, and while other countries are adopting similar standards, the verification infrastructure does not exist everywhere yet.
Some Indian carriers, for example, run their own robocall prevention policies that produce high block rates for international VoIP calls regardless of attestation. It is not that the calls are spam. It is that carrier-side filtering treats international VoIP with blanket suspicion. This is an industry-wide challenge, not something specific to FreeCallMe or any single provider.
What to do if your calls are getting blocked
If you are a business or frequent caller running into blocks or spam labels, four moves help the most.
Register your number.
The Free Caller Registry at freecallerregistry.com lets you register your business name and number with T-Mobile and Verizon Wireless directly. It is free and reduces false flagging.
Use a consistent number.
Frequent changes to the originating number look suspicious to analytics platforms. Calling steadily from a single number builds a trust score over time.
Reduce call frequency.
High volume from one number over a short window is the main behavioral signal for spam detection. Spacing calls out helps.
Contact carrier reputation programs.
Verizon, T-Mobile, and AT&T each run processes for businesses to whitelist their numbers. It takes time, but it is the most direct fix for persistent blocking.
For personal calls through FreeCallMe's dialer, the numbers are managed by FreeCallMe and already carry the appropriate registrations, so there is nothing you need to do.
Common questions
- Why does my VoIP call show up as "Spam Likely"?
- Usually because the call arrived with weak or missing STIR/SHAKEN attestation, or from a number the carrier's analytics do not yet trust. The receiving carrier checks how strongly the call is authenticated and how the number has behaved, and when either looks weak it adds a warning label or blocks the call.
- What is STIR/SHAKEN?
- It is the framework US carriers use to digitally sign calls so the receiving network can tell whether the caller is authorized to use the number they are showing. It replaced a system that simply trusted whatever number was attached, which is what made caller ID spoofing so easy.
- Does Level A attestation guarantee my call will not be flagged?
- No. Level A is the strongest signal an originating provider can send, and it makes flagging far less likely, but each carrier and its analytics partners make their own independent decisions based on behavior too. Attestation is the part a VoIP provider controls; the final call is the receiving carrier's.
- Why are calls to India blocked even when they are legitimate?
- STIR/SHAKEN was built for the US and Canada, and the verification infrastructure does not exist everywhere. Some Indian carriers run their own robocall filtering that treats international VoIP calls with blanket suspicion, so calls can be blocked regardless of attestation. This is an industry-wide challenge, not specific to any one provider.
- Can I fix a number that is being flagged as spam?
- Often, yes. Register it with the Free Caller Registry, call from a consistent number, keep the volume reasonable, and use the carriers' business reputation programs to request a review. For personal calls through FreeCallMe's dialer, the numbers are managed by FreeCallMe and already carry the right registrations, so there is nothing you need to do.
The bigger picture
STIR/SHAKEN is genuinely working. Robocall complaints have fallen measurably since it rolled out. The downside is that some collateral damage lands on legitimate VoIP calls that have not been verified. The framework trades a few real robocalls getting through for some legitimate calls getting flagged, and most carriers accept that trade. It is why VoIP calling is not quite as simple as "dial any number in the world." There is a whole trust infrastructure deciding whether your call counts as legitimate, and getting onto the right side of it is the difference between calls that connect and calls that do not.
Place a call that is built to connect
FreeCallMe's dialer sends Level A attestation and registers its numbers. Start with $1 of free credit.
Call any phone number in 220+ countries from your browser. Your first $1.00 is free, then pay-as-you-go by the minute.