Linux detaches modem unexpectedly
 

Hi,
I have a Siemens HC25 modem on my board on which there is the following linux version
Linux version 3.13.6-porteus (root@porteus) (gcc version 4.8.2 (GCC) ) #2 SMP PREEMPT Fri Jun 13 15:44:09 CEST 2014

This modem, when the OS boots, it's configured as a mass storage, after several attempts we managed to turn it into modem mode.
lsusb shows correctly the device:

Bus 003 Device 007: ID 0681:0047 Siemens Information and Communication Products


But, after sometime Linux recognizes it as mass storage again
(this happens pure randomly, sometimes it is minutes others is hours).
Some say this happens when the sub device is idle, in this case it is powered off. Actually, it is not powered off because I can list it with lsusb and unfortunately it's product id is the one used when in mass storage modem.

Bus 003 Device 007: ID 0681:0041 Siemens Information and Communication Products

How can I avoid that the usb device turn into a mass storage? Why it happen?
Thank you in advance.

There should be a message in the kernel message buffer when that happens. Run dmesg immediately after you detect this change and look for a recent related message. Or check the system log (/var/log/messages or /var/log/syslog on most distros).