Had the same problem recently but after a lot of research I nearly gave up, I tried everything even reinstalling modem drivers etc but of course the problem was not modem drivers. I then tested the modem with an earlier build of Windows 10, the modem was working perfectly only the anniversary update (build 1607) broke down the modem functionality.
The solution was to remove and reinstall the wan miniports drivers and rebooting the machine and the modem works flawlessly now in 1607 build. These wan miniports are found by going to device manager and clicking view to enable hidden items. After removing reboot your machine, the install, reboot again they will start working