When you use the Microsoft Skype for Business mobile applications to search a contact, you may find the contact details like person's name, phone number, and so on, cannot be shown in the search result. This issue occurs when the contact SIP address's domain isn't your Skype for Business Server 2015 Active Directory domain. For example, your Skype for Business Server domain is contoso.com, the affected contact's SIP address is email@example.com.
This issue occurs because the Skype for Business lookup mechanism for ABS/DLX services for a federated contact is failed so that UCWA skips the displayName property and other properties.
To fix this issue, install the July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Web Components Server.