If your contact rate has been falling week over week on a list that used to work, this is almost always the reason. Nothing about your agents changed. Your number's reputation did.
What a DID is
A Direct Inward Dialling number is a real telephone number routed to your system. Call centers use them two ways:
- Inbound — the number customers call.
- Outbound caller ID (CLI) — the number you present when calling out.
Typical cost is $1-3 per month, sometimes with a small per-minute inbound charge. Geographic numbers (matching a real area code), national numbers, and toll-free numbers have different costs and different perceptions.
How spam labelling actually works
Carriers and third-party analytics providers score every calling number continuously. The signals:
| Signal | Effect on reputation |
|---|---|
| High call volume from one number | Negative, especially if sudden |
| Very short average call duration | Strongly negative — the classic robocall signature |
| Low answer rate | Negative |
| Consumer complaint reports | Strongly negative |
| Number cannot receive inbound calls | Strongly negative |
| No CNAM registration | Mildly negative |
| Low STIR/SHAKEN attestation | Increasingly decisive |
| Sudden change in calling pattern | Negative |
| Long, answered calls | Positive |
Notice the pattern being detected: many short unanswered calls from a number nobody can call back. That is the fingerprint of a robocall, and a legitimate campaign that dials hard from a single unreachable number produces exactly the same fingerprint.
STIR/SHAKEN in brief
In North America, originating carriers cryptographically sign calls with an attestation level:
| Level | Meaning |
|---|---|
| A — Full | Carrier verified the caller and their right to use the number |
| B — Partial | Carrier knows the customer but not their right to that number |
| C — Gateway | Carrier merely passed the call along; no verification |
Terminating carriers use this in blocking decisions. Level C traffic is increasingly blocked outright. Level A is what you want, and you get it by presenting numbers legitimately assigned to you by the carrier originating your traffic.
This is the technical reason "neighbour spoofing" — presenting a number matching the target's area code that you do not own — has stopped working. It cannot achieve full attestation, so it fails at exactly the point it needs to succeed.
Buying and using DIDs properly
Choose the right type
| Type | Best for | Note |
|---|---|---|
| Local geographic | B2C outbound in a defined region | Higher answer rates when genuinely local |
| National | Brand consistency | Neutral perception |
| Toll-free | Inbound customer service | Poor outbound answer rates; reads as commercial |
| Mobile | Some markets, notably parts of Asia and Africa | Often much higher answer rates locally |
Size the pool
A reasonable starting ratio is one outbound CLI per 3-5 agents. This spreads daily volume so no single number looks like a machine.
What matters is that this is volume distribution, not evasion. The distinction is visible in the data: legitimate rotation keeps stable numbers with consistent patterns and real inbound routing. Evasion cycles rapidly through disposable numbers.
Route every outbound number inbound
Point each CLI at something. Options, in order of quality:
- A live queue where someone answers.
- A voicemail identifying your company and offering removal.
- At minimum, an IVR that states who you are.
Register CNAM where available
In North America, CNAM controls the name displayed alongside the number. Registering your business name is inexpensive and reduces the "unknown number" penalty.
Warm new numbers up
Do not put a brand-new DID straight onto 500 calls a day. Ramp over a week or two. A number that goes from zero to high volume overnight looks like exactly what analytics engines are built to catch.
Diagnosing a reputation problem
Symptoms, roughly in order of appearance:
- Contact rate falls steadily over one to three weeks on an unchanged list.
- Agents report prospects saying "it came up as spam."
- Calls to certain carriers stop connecting at all while others work.
- ASR drops on one destination while others are unaffected.
Checks:
- Call your own CLI from a mobile on each major carrier. See what displays.
- Use the free lookup tools the major US carriers provide for checking and reporting number labelling.
- Compare answer rate by CLI in your dialer reports. If one number's rate collapsed while others held, it is the number.
- Check average call duration. If it is very low, you are generating the signal yourself.
Recovering a flagged number
- Stop using it immediately for high-volume outbound. Continued volume reinforces the label.
- Fix the underlying cause — route it inbound, register CNAM, reduce per-number volume, improve list quality so calls last longer.
- File remediation requests with the analytics providers. The major ones operate free portals for legitimate businesses to dispute labelling.
- Wait. Reputation recovers over weeks, not hours.
- Reintroduce slowly at low volume.
If a number has been heavily flagged, replacement is often faster than rehabilitation — but only if you fix the cause first. Otherwise the replacement is flagged too, and now you have burned two numbers.
What not to do
- Do not spoof numbers you do not control. It fails attestation, it is an enforcement target under the US Truth in Caller ID Act, and it stopped working technically.
- Do not cycle through disposable numbers to evade labelling. Detected, treated as an evasion signal, and increasingly results in range-level blocking.
- Do not present a number that rings nowhere.
- Do not ignore complaint reports. They are the heaviest negative signal, and each one represents a compliance risk as well as a reputation hit.
Where to go next
- VoIP fundamentals: SIP, codecs, and DIDs — the underlying concepts.
- Compliance: TCPA, DNC, and GDPR — caller ID law.
- Call center KPIs that actually matter — tracking contact rate by CLI.
Frequently asked questions
Why are my calls showing as Spam Likely?
Carrier analytics engines score every number on call volume, call duration, answer rate, complaint reports, and whether the calling number is reachable for return calls. Short unanswered calls at high volume from a number that cannot receive calls is the classic spam pattern, and it will be labelled within days regardless of how legitimate the campaign is.
Does rotating caller ID numbers avoid spam labelling?
Rotating a small pool across legitimate traffic to spread volume is a normal practice. Rapidly cycling through many numbers to evade labelling is detected, treated as an evasion signal, and increasingly results in whole ranges being blocked. It also fails STIR/SHAKEN attestation, which makes the problem worse rather than better.
What is STIR/SHAKEN attestation?
A framework used in North America where the originating carrier cryptographically signs a call with an attestation level. Full attestation (A) means the carrier verified both the caller identity and their right to use that number. Lower levels signal less certainty and increasingly attract blocking or labelling downstream.
How many DIDs does a small call center need?
For inbound, one per campaign or client is usually enough. For outbound caller ID, a common ratio is one number per 3 to 5 agents so that no single number carries excessive daily volume, with each number pointed at a real inbound destination.