Symptoms
Consider the following scenario:
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User A is Enterprise Voice enabled in Microsoft Skype for Business Server 2015. User B is a PBX user who is configured to use a non E-164 number in the PBX/Gateway that's connected to the Skype for Business Mediation Server.
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User A uses the Skype for Business for Mac, Skype for Business for iOS, or Skype for Business for Android client to start a call to user B through a non-E.164 (international public telecommunication number format) telephone number.
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User B answers the call successfully.
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User A hangs up the call. Then the call is saved in conversation history under the Calls tab.
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User A tries to call the telephone number again.
In this situation, the call fails.
Cause
This issue occurs because the call that's saved in conversation history doesn't have the "SIP URI user=phone" parameter string of the INVITE TO header. This causes the call not being routed to the destination telephone number.
Resolution
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.
