On most Qualcomm SoCs the modem is a separate processor which lives next to the main CPU where Linux is running on. It needs to be given firmware by the host processor to start.
The procedure for getting the modem to work is generally the following:
- Package and install the Firmware of your modem into the file system, generally the
- Install the package
msm-modem-mainline, depending on the kernel you are using (and reboot to activate the udev rules and services).
ofonoand start it using
rc-service ofono start
- You should see signs of your firmware being loaded in the dmesg.
- You can use the ofono test scripts or
ofonoctlto see if the Linux recognized your modem and if it works correctly.
- Note, that for dual SIM (maybe only for DSDS - dual SIM dual standby?) devices you need to follow additional manual steps, otherwise the modem (and ofono) will not recognize your SIM card. See 
- #520 N900: integrate ofono
- #598 Let's get the cellular modem working! (presenting various stacks, general discussion)
- #1054 Get qualcomm modems working (at the end of the thread scintill wrote a few wrappers and made SMS on a QC modem work with a downstream kernel without userspace blobs!)
- #1314 i9195/Qualcomm modem support (scintill's work integrated)
- pmaports#251 xiaomi-santoni: gets modem to work on the device
- Maemo Leste bugtracker#27 Call/Phone support