Symptoms
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.
Resolution
To fix this issue, install one of the following updates:
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The September 2020 cumulative update 7.0.2046.244 for Skype for Business Server 2019 and Unified Communications Managed API 5.0 Runtime.
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The May 2020 cumulative update 6.0.9319.580 for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime.
Third-party information disclaimer
The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.