How to block inbound spam/spoofing calls into Microsoft Teams

Every year people get impacted by the spam call is growing exponentially. Truecaller announced research conducted in partnership with The Harris Poll in March of 2022, and the findings detail trends/insights on the impact of spam and phone scams that have increasingly permeated the U.S. over the last 12 months. The study estimates that a staggering $39.5 billion was lost to phone scams this past year, the highest number recorded since Truecaller began researching scam and spam calls in the U.S. eight years ago.

Sometimes spammers use/spoof your organization numbers to call someone from PSTN. We used to block inbound spam/spoofing calls in Cisco using the route pattern option. If you have used AudioCodes or other SBC, you might have configured specific IP routing tables to block the numbers.

Since many organizations are moving their telephony to Microsoft Teams either through Direct Routing, Operator Connect, or Microsoft Calling Plan, we need to ensure we protect our environment.

Microsoft Teams has the option to block inbound calls in different ways,

• Enable Spam filtering in the Teams Calling Policy.

• Block the numbers based on the number pattern.

• Block Inbound Calls in Teams client by a specific number.

• Block all unknown caller-id calls.
• Block inbound calls in SBC (if you are using Direct Routing)

Option: 1 - Enable Spam filtering in the Teams Calling Policy - 

Spam filtering has been available within Teams, and it uses Microsoft's algorithms and data to determine a spam score for incoming PSTN calls. This feature detects the suspicious incoming call, and it will header the call toast with "Spam likely," allowing the recipient the chance to decline the potential spam call.

This feature is enabled by default on the Microsoft Teams Global Calling Policy. You should adjust the settings if you have a custom policy created.

The Policy has three options,

• Turn on - When this option is set, the basic and captcha interactive voice response checks are performed.

• Turn on without IVR - Only basic checks are performed. However, if the Spam call score is high, the call will get dropped.

• Turn off -  Spam filtering is turned off.

Option:2 - Block the numbers based on the number pattern -

Your admins can create a tenant-level inbound block number pattern to block incoming calls with a specific number or range of numbers.

In this example, I want to block PSTN inbound calls using my organization's number as source/caller,

New-CsInboundBlockedNumberPattern -Name "Block_CA_Range" -Enabled $True -Description "Block California Range numbers" -Pattern "^\+?1612532XXXX$"

When the block number pattern is configured, all the inbound calls have checked this list before the routing decision. If the number is found in the list, the calls will be rejected, else the call will be routed to the user.

Block individual Number: -

You can configure individual numbers as well.

New-CsInboundBlockedNumberPattern -Name "Block_Spam_Call" -Enabled $True -Description "Block Spam call" -Pattern "^\+?16125322222$"

Create an Exception pattern/individual number to allow the call -

You can also configure exceptions in the range if you would like to route/allow a specific number,

New-CsInboundExemptNumberPattern -Name "Allow_CA_Support_Number" -Enabled $True -Description "Allow Suppport Number" -Pattern "^\+?16125321111$"

View all blocked numbers in your tenant -

You can find all blocked numbers using the,

Get-CsInboundBlockedNumberPattern

Option -3 Block Inbound Calls in the SBC: -

In this example, I'm using AudioCodes SBC configured for Direct Routing to block the spoofing calls before they get routed to Teams.

Option: 4 - Block Inbound calls in the Teams Client -

On the Teams client, navigate to Settings, select Privacy, and select the "Block calls with no caller ID" option.

By default, this option is not selected.

Block Individual Number from Teams Call History Tab -

On the Teams client, navigate to Calls Tab, select the Inbound call and select the (...) option, and select "Block".