Resuming a PSTN call that was put on hold results in one-way audio in Skype for Business Server 2019 and 2015

Symptoms

In a Microsoft Skype for Business Server 2019 or Skype for Business Server 2015 environment, consider the following scenario:

A PSTN user starts a call with a Skype for Business client user. The connection is established, and the audio is fine.
The Skype for Business client user places the call on hold.
The Skype for Business client user resumes the call.

In this situation, the PSTN user can’t hear the Skype for Business client user, but the Skype for Business client user can hear the PSTN user.

This issue occurs because the Skype for Business Mediation Server can’t correctly process the SIP SDP message that’s received from the gateway.

To fix this issue, install one of the following updates:

For Skype for Business Server 2019, install the December 2019 cumulative update 7.0.2046.151 for Skype for Business Server 2019, Mediation Server and the December 2019 cumulative update 7.0.2046.151 for Skype for Business Server 2019, Web Components Server.
For Skype for Business Server 2015, install the July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015, Mediation Server.

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