Modem Driver installed under Windows 10 Build 10240 and before has an Unimodem Audio Device which appears under Device manager and supports to Voice function properly.
However, once upgrade to Windows 10 Build 10586, Unimodem Audio Device Is removed and no Voice function is available
This is a known issue. See here.
Hiro Inc. will discontinue modem voice support after upgrade to Windows 10 Build 10586
We are not able to resolve this isuse and encourage users to reply and respond to the thread in order to increase MSFT's attention, hope MSFT will fix this issue in the next Build.