Consider the following scenario:
User A starts a call with user B in Microsoft Skype for Business 2015 (Lync 2013), and user B accepts the call.
User A tries to transfer the call to user C who is offline.
User C does not accept the call.
User A receives a "Cannot complete the transfer" error notification.
User A clicks the notification to see the details of the error.
The error details show that "Call was not transferred because user A cannot be reached and may be offline".
In this scenario, user C should be shown in the details of the error.
To fix this issue, install the March 6, 2018, update (KB4018290) for Skype for Business 2015 (Lync 2013).
Note Lync 2013 was upgraded to Skype for Business in April 2015.