In Microsoft Skype for Business Server 2015, you enable the Busy on Busy feature for a user so that new incoming calls will be rejected with a busy signal if the user is busy. However, this user can't receive the new incoming calls if the previous calls doesn't expire correctly.
This issue occurs because the Session-Expires header misses the HeartBeatInterval value. This causes the expired call can't be removed from Skype for Business call store.
To fix this issue, install the July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Front End Server and Edge Server.