When installing the MT9234MU-CDC-XR modem from Multi-Tech in 64 bit versions of Windows 10, the modem is not detected as a Unimodem Audio Device. Since it is not detected as an audio device, the voice functionality of the modem is not supported. The issue is not unique to Multi-Tech modems, all TAPI modems utilizing the Unimodem Audio Device were affected.
Windows 10 builds prior to 1511 were functioning normally with TAPI modems and the Unimodem Audio Device would be recognized and installed correctly. Once Microsoft released build 1511, TAPI modems utilizing Unimodem Audio Devices stopped functioning.
On December 9th, 2016, Microsoft released an update which resolves the issue with the Unimodem. You can find details about the update on Microsoft's website here, https://support.microsoft.com/en-us/kb/3201845.
In order to receive the update, run Windows Update and be sure to select KB3201845.
You can check for the correct version of Windows 10 by verifying the build is 14393.479 or higher.
Once the update is installed, unplug the modem from the PC and plug it back in. Open Device Manager and verify the Unimodem Audio Device was installed, you should see it under Sound, video and game controllers as Unimodem Full-Duplex Audio Device.
If the Unimodem is present but you're still not able to hear audio on phone calls, try restarting the Telephony Windows service or reboot the PC.
If you are still experiencing issues with your modem after updating Windows 10 and rebooting your PC, please submit a support request or contact Product Support at 512-326-1011 x3 or toll free in the US and Canada at 1-800-331-8740 x3