Symptoms
Microsoft Skype for Business for iOS and Android mobile clients can't show the display name of a federated contact in the contact list. Additionally, users may see only the SIP address displayed in there. It's expected that the mobile clients can show the display names exactly like the Skype for Business 2015 (Lync 2013) or Skype for Business 2016 desktop client for the federated contacts.
Resolution
To fix this issue, install the July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Web Components Server.