How to block unwanted callers — blacklist and blocked numbers

What it actually is

Every organization knows them: a sales agent who calls three times a day, a pesky caller, an automated recording offering an "instant loan", or a person who has already been told there is nothing to help them with and keeps calling anyway. Each such call ties up an employee, disturbs, and sometimes is real harassment.

Caller blocking is a general name for all the ways of handling this at the system level, before the call reaches a person. The common form is a blacklist (Blacklist): a list of numbers that the PBX recognizes and handles differently from other calls.

Blocking relies on caller ID — the number that arrives with the call. Hence its limit: it works well against someone who calls from the same number, and less well against someone who changes numbers or hides them.

In the past, on regular phone lines, blocking was a service you asked for from the phone company, and sometimes paid for. In a cloud PBX it is part of the organization's own settings: you add a number, set a destination for it, and the change takes effect immediately — without waiting for anyone. And just as easily you can cancel it, if it turns out the block was a mistake.

What you can do with a blocked call

"Blocking" does not have to mean hanging up. There are several options, and each suits a different situation:

The last option is surprising, but very useful: not every problematic caller needs to be blocked. Sometimes it is enough for them to reach the person who knows the matter, and not the new secretary.

Withheld numbers

A call from a withheld number is a call where the caller asked that their number not be displayed. On the display you see "Withheld", "Private" or "Anonymous". There are entirely legitimate reasons for this — doctors, social workers and people calling from an institution's phone hide the number to protect privacy. But it is also the preferred way of pesky callers.

In many PBXs you can decide what to do with withheld-number calls as a group: block all of them, send them to a message asking them to call with a visible number, or let them ring as usual. This is called Anonymous Call Rejection.

Before blocking, think about who might be hurt. In an educational institution, for example, a parent calling from a work phone may appear as withheld. A good middle solution is a message: "Calls from a withheld number are not accepted, please call from an identified number". That way, someone who really needs to knows what to do.

Whitelist: the opposite of a blacklist

Sometimes it is easier to define who is allowed, rather than who is not. This is a whitelist (Whitelist): only numbers on the list reach the destination, and everyone else gets a different destination. This suits situations where the circle of callers is defined and small.

A familiar example is a student phone in a dormitory: students can receive calls only from numbers the parents approved in advance. Another example is an internal staff line that only employees are supposed to call.

• Blacklist: everyone gets in, except a few. Suits most businesses.
• Whitelist: no one gets in, except a few. Suits closed and sensitive lines.

Where the blocking happens: on the phone, in the PBX or at the carrier

There are three layers where you can block, and it is worth understanding the difference:

• On the phone itself: most IP phones have an internal blacklist. It blocks only that device — the rest of the office keeps receiving the calls.
• In the PBX: blocking at the organization level. A number blocked here will not reach any extension, and you can set a proper destination for it. This is the recommended way for a business.
• At the carrier or with external tools: some telecom companies offer filtering of numbers that were reported as spam. This is an additional layer, but it is not under your control.

In an organization, blocking on a single employee's phone is usually a temporary solution. If the pesky caller bothers everyone, they will simply call another extension. Blocking in the PBX solves it once for everyone.

With routing by caller: the tool that already exists

In most cloud PBXs, blocking is actually a special case of routing by caller. This tool checks the caller's number and decides where to send the call. It is usually used to give better service — an important customer straight to their account manager, a number from abroad to an English-speaking agent. But the same tool can also send a specific number to a completely different destination: a message, a voicemail box or a hang-up.

The advantage of this approach is that everything is in one place. The decision "what happens when this number calls" sits in the same place where all the other routing is defined, and you can see it in the full picture of the call path.

The limits of blocking

It is important to be realistic: blocking by number does not stop everything.

• Changing numbers: a lot of spam calls come each time from a different number. A blacklist will not help against them — every new number gets through.
• A displayed number that is not real: some callers display a number that is not theirs. In such a case the block may hurt the real owner of the number.
• A determined person: someone who really wants to get through will call from another phone.

So blocking is a good tool against a repeating, constant nuisance, and less so against a changing one. Against many automated calls, other tools work better — for example a short IVR menu at the entrance ("To speak with the office, press 1"), which automated systems usually do not get past.

Real harassment

When it is not a nuisance but harassment or a threat, blocking is only part of the solution. In such a case it is important not to delete the traces: save the call details from the call log, and the recordings too, if there are any. In serious cases you turn to the police, and this documentation helps.

For this very reason, it is sometimes better to send such a caller to a voicemail box rather than hang up on them: that way a record of what they say is kept, without any employee having to hear it in real time.

An example from the field

In the office of the "Maos Chitim" charity fund (a pseudonym), someone started calling dozens of times a day, always from the same number, and shouting at the secretaries. The staff blocked him on their phones — and he moved to the treasurer's extension, and then to the fax extension.

The solution was to set his number in routing by caller in the PBX, and send him to a dedicated voicemail box. The calls stopped reaching the staff, the messages were saved, and the coordinator went through them once a day to make sure there was nothing really important in them. After two weeks the calls stopped.

Managing the list over time

A blacklist tends to grow and be forgotten. After a year it has dozens of numbers, and nobody remembers why each one went in. The problem shows up on the day a new supplier, who got a number that once belonged to a sales agent, cannot reach you.

A few habits that prevent this:

• Write a reason and a date next to every blocked number — "sales call center, Nisan", "harassment, reported to the manager".
• Go over the list every few months and delete old numbers that no longer call.
• One person in charge: only one or two people should add numbers. When every employee blocks by feel, customers who were simply upset once get blocked too.
• Prefer a soft destination: a voicemail box instead of a hang-up, when you are not sure. That way a mistake does not cost you a customer.

Blocking and service: where the line is

Blocking is a powerful tool, and it should be used with care. A service organization, and especially a public institution or nonprofit, wants everyone who needs help to be able to reach it. A caller who calls often is not always harassing — sometimes they simply did not get an answer, and feel that nobody hears them.

So before blocking, it is worth asking: can the matter be solved? One conversation with a person in charge, explaining what is possible and what is not, often stops a series of dozens of calls. Blocking fits when the call is no longer related to the service — automated recordings, sales, or abusive behavior.

Robocalls and sales call centers

A large share of unwanted calls to businesses comes from automatic dialing systems: a recording that starts talking the moment you answer, or silence for a few seconds until an agent joins. Against these, a blacklist helps only partly, because the numbers change.

What works better is not letting them reach a person in the first place. At the entrance to the main line, a short message asking to press a digit to continue filters most automated calls — and a real caller hardly notices. At the same time, it is worth asking regular call centers to remove your number from their dialing lists. Most orderly call centers honor such a request.