First, you need to check if your modem support voice calls. 

Connect in the TTY (In FreeBSD i use, cu -l /dev/ttyUx.0), and send this command

AT^CVOICE=?

Then, the return must be something like: ^CVOICE:(0)

To enable voice, send AT^CVOICE=0
To disable voice, send AT^CVOICE=1

If this wont work, and the voice is disabled, try to use dc-unlocker

https://freeswitch.org/confluence/display/FREESWITCH/mod_gsmopen#mod_gsmopen-VoicecallsandSMS

Try to make a call, and send the log during the call.

You need to remove and reload this modem before, because when it enter in this state, you was unable to do calls again.

The list before this test, must be something like this, note State = IDLE and CallFlw = CALL_IDLE.

freeswitch@internal> gsm list
gsm console is NOT yet assigned
F ID Name       Operator         IMEI            IB (F/T)  OB (F/T)  State   CallFlw         UUID
= == ========== ================ =============== ========= ========= ======= =============== ====
  1  gsm_1      72416            359390027149600    0/0       0/0    IDLE    CALL_IDLE
  2  gsm_2      72404            353871025084879    0/0       0/0    IDLE    CALL_IDLE
  3  gsm_3      72406            354112034907191    0/0       0/0    IDLE    CALL_IDLE
  4  gsm_4      72405            867919018470461    0/0       0/0    IDLE    CALL_IDLE

So, make a call test, and report the log, to see whats happening.

--
Luiz Fernando Softov
+55 67812 34276
http://www.softov.com.br
fernando@softov.com.br