I've just upgraded the firmware on my Cell-C E3276-150 from 1geoff99's links
here.
The modem was supplied with firmware version 21.192.13.00.00.
First I upgraded to
Huawei E3276s-920 21.260.03.00.00 normal.
Then I upgraded to
Huawei E3276s-861 21.436.03.00.00 Universal.
It appears that the firmwares of the E3276 are interchangeable and you do not have to find one that matches your exact model number, e.g. -150.
^DIALMODE has now changed from 1,2 to 2,3 meaning both Modem and NDIS modes support CDC specification.
The device now appears as a serial modem and a network interface in Windows.
In Ubuntu Trusty, the modem now works with the stock kernel and ModemManager "out-of-the-box". With
Modem-Manager-GUI (in the Ubuntu repositories) I can check signal strength and check balances with USSD, SMS, etc.
Edit: After upgrading the firmware I found the connection would go dead in Windows after a several minutes; in Linux it was fine for hours.
I upgraded my Huawei drivers from 4.25.11.0 to 5.01.06.00 (available
here - thanks 1geoff99!) and all seems to be well.