You can add a user’s telephone number through the Microsoft Office 365 portal or in the on-premises Active Directory Domain Services (AD DS), if the Directory Synchronization Tool is configured. After 24 hours, the change will be visible in the Microsoft Lync contact card for the user.
Read more at source: https://blogs.technet.microsoft.com/latam/2016/03/25/how-to-add-phone-numbers-to-skype-for-business-online-users/