Hi Karim Aly,
In order to narrow down the issue, please help us confirm the following questions:
1. Did you mean AV conference call or PSTN calls?
2. When you make a phone call to another SFB client, the others can pick up the call but he/she can’t hear the voice, is that right?
3. Did the issue only appear on the specific SFB client?
Regarding this issue, please check if there is Event ID in SFB FE server which suggested by Shankar.
If the issue only appeared on the specific SFB client, please try to delete SFB cache files and test again. (%userprofile%\AppData\Local\Microsoft\Office\15.0\Lync\sip_UserName@Domain.com)
If the issue is about PSTN calls, please check the configuration of Enterprise Voice, the following blog is for your reference
https://technet.microsoft.com/en-us/library/gg412876.aspx
Regards,
Alice Wang
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