$ lsusb
(...)
Bus 003 Device 004: ID 05ac:12ac Apple, Inc. iPhone 5/5C/5S/6/SE/7/8/X/XR (backdoor ssh mode)
(...)

$ sudo nano /etc/udev/rules.d/99-gsm.rules

  GNU nano    /etc/udev/rules.d/99-gsm.rules

SUBSYSTEM=="tty", ATTRS{idVendor}=="05ac", ATTRS{idProduct}=="12a8", MODE="0666", SYMLINK+="ttyGSM", RUN+="/bin/systemctl restart smstools"

$ sudo nano /etc/smsd.conf

  GNU nano    /etc/smsd.conf

#
# /etc/smsd.conf
#
# Description: Main configuration file for the smsd
#

devices = GSM1
outgoing = /var/spool/sms/outgoing
checked = /var/spool/sms/checked
incoming = /var/spool/sms/incoming
logfile = /var/log/smstools/smsd.log
infofile = /var/run/smstools/smsd.working
pidfile = /var/run/smstools/smsd.pid
failed = /var/spool/sms/failed
sent = /var/spool/sms/sent
stats = /var/log/smstools/smsd_stats
loglevel = 7
# autosplit 0=no 1=yes 2=with text numbers 3=concatenated
autosplit = 1

[GSM1]
device = /dev/ttyGSM
incoming = yes
init = AT+CPMS="SM","SM","SM";+CNMI=2,0,0,2,1