Comment 58 for bug 456612

I had face this similar issue with Zte Ac2787 CdMA 3g device.
And the system did not detect modem in network manager
Used to connect with gnome-ppp
Its may be strange or weird but couple of days ago I followed the following steps and its now detects the device smoothly\\

Installed wvdial ( I don't think needed but its in the steps I followed)
fired commands in terminal
$ nm-online
$ nm-tool
and boom it showed that the devise detected.
and when you can also see it in the network list under mobile broadband in network manager from system tray.
Connectly smoothly even after reboot.
Repeated this step on 2 ubuntu and 2 linux mint machines worked on all.
weird but worked for me .