If your company is set up to support federation, you can add Skype for Business users at other companies.
When communicating with someone at another company, you can use only the Skype for Business features (for example, video calls or desktop sharing) that are enabled at both of your companies.
Support for having contacts who use instant messaging (IM) providers other than Skype for Business isn’t included in Skype for BusinessOffice 365 operated by 21Vianet.
Note: To learn more about support for external contacts at your company, contact your workplace technical support.
To add an external contact
In the upper-right corner of your Contacts list, click the Add a contact icon > Add a Contact Not in my Organization > Skype for Business, and then provide the required details. For details, see Add a contact in Lync.