Create information barrier segments first, then create information barrier policy based on the segments then start the information barrier application.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/compliance/information-barriers-policies

Explanation:

According to the Microsoft documentation1, you need to add an emergency address for each location where your users make and receive phone calls. An emergency address is a civic address-the physical or street address of a place of business for your organization. It is used to route emergency calls to the appropriate dispatch authorities and to assist in locating the emergency caller.
After you add an emergency address, you need to assign a phone number to it. You can assign either a user number or a service number. A user number is assigned to a specific user in your organization who makes and receives phone calls. A service number is used for services such as Audio Conferencing (for dial-in numbers), auto attendants, or call queues.
To assign a phone number to an emergency address, you need to modify the location for each phone number.
You can do this by using the Microsoft Teams admin center or PowerShell. For example, to modify the location for a user number by using PowerShell, you can use the Set-CsOnlineVoiceUser cmdlet with the - LocationID parameter2.