quoted

But the problem here is that pl2303 should not be loaded at all as
it's for USB-serial adapter, not 3G modem. I'm not sure what controls
that.

The descriptors of the device itself.

What does 'lsusb' show with this device plugged in?
And what kernel version are you using here?

I'm using 2.6.21 on an ARM board. lsusb says:
|12d1:1446
after usb_modeswitch:
||12d1:1001
|