In Microsoft Skype for Business Server 2019 or Skype for Business Server 2015, when you have a conference call with a user who joins the call by using a Cisco phone, the call will be unexpectedly disconnected after 28 or 56 minutes.
To fix this issue, install one of the following updates:
The September 2020 cumulative update 7.0.2046.244 for Skype for Business Server 2019 and Unified Communications Managed API 5.0 Runtime.
The May 2020 cumulative update 6.0.9319.580 for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime.
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