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3CX Month: How to Blacklist Phone Numbers or IP Addresses

Every business day for the month of November, we will be posting a useful tip for 3CX, a powerful SIP-based unified communications platform.

Your 3CX phone system is a powerful tool for making you available for colleagues, clients and customers around the world.

Sometimes, however, available is the last thing you want to be. Robo-callers and solicitors can take up a huge amount of time with their pointless calls. Even worse, hackers try to attack your phone system to gain access to your information and cause havoc.

The solution: Blacklisting.

Blacklisting means your phone system will automatically reject a phone number or IP address when it tries to contact you.

Blacklisting Phone Numbers in 3CX

Blacklisting phone numbers can save you from persistent telemarketers or from anonymous robo-calls. No, you don’t have time for that survey!

You can either blacklist by phone number or by caller ID.

In the 3CX Management Console, go to Settings > Blacklisted Numbers and click “Add.”

There will be two fields to fill out: “Incoming caller ID to be blocked” and “Description.”

Under the first, enter the full phone number or caller ID that you want to block. Note that they must match precisely, including case: “jeremy” and “Jeremy” are considered to be different. (You can use an asterisk to cover both situations: “*eremy”.)

Make sure you pay attention to what information your phone service provider displays for you, because the information needs to be exact. For example, some providers display the country prefix (in the US, the “1” before a 10-digit phone number), while others don’t. If your provider does, you need to as well. The same precaution applies to blocking by caller ID: it must be exact, or it won’t work.

The “Description” field is simply for you to know why you blocked the number.

After you’ve filled the information in, click “Ok” and you’re good to go!

Blacklisting IP Addresses in 3CX

There are two basic methods of controlling access from IP addresses to a computer system: whitelisting and blacklisting.

Whitelisting assumes that nobody is ok unless you say they are: you give IP addresses permission to connect.

Blacklisting, on the other hand, assumes everybody is ok unless you say they aren’t: you block bad IP addresses while letting others connect.

In reality, modern communications systems are so advanced that both methods are used. 3CX even mixes in automatic blacklisting. If an IP address tries to connect too much (a common sign of an attack), 3CX will automatically block that IP address to protect your phone system.

If you want to manually blacklist an IP address, first go to the 3CX Management Console.

Go to Dashboard > Blacklisted IPs, and select “Add.”

From here, you can choose to blacklist either a single IP or a range of IPs. Choose which one you want, enter the appropriate information, and choose the action “Deny.” Enter a description to remind you why you blacklisted the IP address, click “Ok,” and you’re good to go!

Note: If you want to whitelist an IP address, use the exact same method, except choose the action “Allow” instead of “Deny.”