Operating system name and version: openSuSE 42.2
Version of smsd: 3.1.21, 3.1.20 - both versions affected.
Smsd installed from: sources (3.1.21) and package repository (3.1.20)
Name and model of a modem / phone: PCI M.2 modem Sierrawireless EM7455. MC7455 could be affected too.
Interface: serial (USB bus emulation)

Smsd could not connect to the modem:

/var/log/smsd> sudo cat smsd.log 2017-07-03 02:20:30,2, smsd: Smsd v3.1.21 started. 2017-07-03 02:20:30,2, smsd: Running as smsd:dialout (479:16). 2017-07-03 02:20:30,7, smsd: Running startup_check (shell): /var/spool/sms/incoming/smsd_script.THs4qH /tmp/smsd_data.ztKYy0 2017-07-03 02:20:30,7, smsd: Done: startup_check (shell), execution time 0 sec., status: 0 (0) 2017-07-03 02:20:30,4, smsd: File mode creation mask: 022 (0644, rw-r--r--). 2017-07-03 02:20:30,5, smsd: Outgoing file checker has started. PID: 12083. 2017-07-03 02:20:30,7, smsd: All PID's: 12083,12084 2017-07-03 02:20:30,5, GSM1: Modem handler 0 has started. PID: 12084. 2017-07-03 02:20:30,5, GSM1: Using check_memory_method 1: CPMS is used. 2017-07-03 02:20:30,3, GSM1: Couldn't open serial port /dev/ttyUSB2, error: Device or resource busy, waiting 30 sec. 2017-07-03 02:21:00,3, GSM1: Cannot open serial port /dev/ttyUSB2, error: Device or resource busy 2017-07-03 02:21:00,2, GSM1: Modem handler 0 terminated. PID: 12084, was started 17-07-03 02:20:30, up 0 min. 2017-07-03 02:21:00,2, smsd: Modem handler 0 (GSM1) terminated while mainprocess is still running. Exited, status 127.

However I could connect to the modem manually with socat:
/var/log/smsd> sudo socat - /dev/ttyUSB2 ATI ATI ^C /var/log/smsd> sudo socat - /dev/ttyUSB2,crnl ATI Manufacturer: Sierra Wireless, Incorporated Model: EM7455 Revision: SWI9X30C_02.20.03.00 r6691 CARMD-EV-FRMWR2 2016/06/30 10:54:05 MEID: ****** IMEI: ****** IMEI SV: 6 FSN: ******* +GCAP: +CGSM OK

Note that AT commands work only with correct line termination (crnl socat option):
crnl Converts the default line termination character NL (’\n’, 0x0a) to/from CRNL ("\r\n", 0x0d0a) when writing/reading on this channel (example). Note: socat simply strips all CR characters.

Is my problem related to the line termination or something else?
I haven't found anything about line termination characters in smsd configuration manual.
My smsd config is:

devices = GSM1 logfile = /var/log/smsd/smsd.log loglevel = 7 [GSM1] device = /dev/ttyUSB2 incoming = yes baudrate = 115200 eventhandler = /dev/shm/test/sms_event_handler

115200 is the correct baud rate for this modem:

# socat - /dev/ttyUSB2,b115200,crnl AT OK ATI Manufacturer: Sierra Wireless, Incorporated Model: EM7455 Revision: SWI9X30C_02.20.03.00 r6691 CARMD-EV-FRMWR2 2016/06/30 10:54:05 MEID: ****** IMEI: ****** IMEI SV: 6 FSN: ****** +GCAP: +CGSM,+DS,+ES OK

I think that the problem is not related to the line termination. Smsd uses CR only, and it has been enough for all modems so far.

I do not know what is causing the error "Device or resource busy", when at the same time socat works well. I tested this on openSUSE 42.2 with Huawei USB stick and smsd 3.1.21 from sources, and everything worked fine. I think that you could use lsof | grep /dev/ttyUSB2 to get a clue about the process which is using the device.