Stock DigiNet Mobil E3131H USB dongles, 21.6 Mbps, distributed by RCS-RDS Romania http://www.rcs-rds.ro to its residential clients (BLUE modem). At the time of the test (November 11th, 2014), it probably doesn't work just because is „seen” as a mass storage device, not USB modem (in usb_modeswitch).

lsusb

Bus 002 Device 006: ID 12d1:15ca Huawei Technologies Co., Ltd. E3131 3G/UMTS/HSPA+ Modem (Mass Storage Mode)

lsusb -v

Bus 002 Device 006: ID 12d1:15ca Huawei Technologies Co., Ltd. E3131 3G/UMTS/HSPA+ Modem (Mass Storage Mode)
Couldn't open device, some information will be missing
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol       255 
  bMaxPacketSize0        64
  idVendor           0x12d1 Huawei Technologies Co., Ltd.
  idProduct          0x15ca E3131 3G/UMTS/HSPA+ Modem (Mass Storage Mode)
  bcdDevice            1.02
  iManufacturer           1 
  iProduct                2 
  iSerial                 3 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           32
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower              500mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass         8 Mass Storage
      bInterfaceSubClass      6 SCSI
      bInterfaceProtocol     80 Bulk-Only
      iInterface              4 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x01  EP 1 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               1

dmesg

[  265.462978] usb 2-1.1: new high-speed USB device number 5 using ehci-pci
[  265.553614] usb 2-1.1: New USB device found, idVendor=12d1, idProduct=15ca
[  265.553620] usb 2-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[  265.553623] usb 2-1.1: Product: HUAWEI Mobile
[  265.553626] usb 2-1.1: Manufacturer: HUAWEI
[  265.553628] usb 2-1.1: SerialNumber: FFFFFFFFFFFFFFFF
[  265.830391] usb-storage 2-1.1:1.0: USB Mass Storage device detected
[  265.830447] scsi6 : usb-storage 2-1.1:1.0
[  265.830513] usbcore: registered new interface driver usb-storage
[  266.845493] scsi 6:0:0:0: CD-ROM            HUAWEI   Mass Storage     2.31 PQ: 0 ANSI: 2
[  266.846232] scsi 6:0:0:1: Direct-Access     HUAWEI   TF CARD Storage  2.31 PQ: 0 ANSI: 2
[  266.850201] sr1: scsi-1 drive
[  266.850383] sr 6:0:0:0: Attached scsi CD-ROM sr1
[  266.850496] sr 6:0:0:0: Attached scsi generic sg3 type 5
[  266.850774] sd 6:0:0:1: Attached scsi generic sg4 type 0
[  266.854553] sd 6:0:0:1: [sdc] Attached SCSI removable disk