Symptoms
In a Microsoft Skype for Business Server 2015 environment, you make an outgoing PSTN call by using the Skype for Business client. When the called party answers the Session Description Protocol (SDP) offer from the Skype for Business client with G.711alaw and G.711ulaw but puts G.711alaw on the top to indicate preference for G.711alaw, Skype for Business Mediation Server ignores this and sends G.711ulaw. This results in asymmetric codec and one-way voice for PBXs or phones who can't handle the asymmetric payload.
Resolution
To fix this issue, install the July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015, Mediation Server.
