Explanation/Reference:
Explanation:
Allow users to contact external Skype for Business users
Use the steps in this article when:
You have users on different domains in your business. For example, [email protected] and

[email protected].
You want the people in your organization to use Skype for Business to contact people in specific

businesses outside of your organization.
Enable business-to-business communications for your users
1. Sign in with your Office 365 admin account.
2. In the Office 365 admin center, go to Admin Centers > Skype for Business.
3. In the Skype for Business admin center, choose Organization > External communications.
4. To set up communication with a specific business or with users in another domain, in the drop-down box, choose On only for allowed domains.
OR, if you want to enable communication with everyone else in the world who has open Skype for Business policies, choose On except for blocked domains. This is the default setting.
References: https://docs.microsoft.com/en-us/skypeforbusiness/set-up-skype-for-business-online/allow- users-to-contact-external-skype-for-business-users