It seems the /dev/tty* address for the attached USB is changing intermittently, giving a permanent rules name for each device will be the workaround. Based on this http://hintshop.ludvig.co.nz/show/persistent-names-usb-serial-devices/

So the udev rules for cocemsd USB datacard, config file under folder /etc/udev/rules.d, with file name 90-phone.rules, is edited with this contents

KERNEL=="ttyUSB*", ATTRS{idVendor}=="12d1", ATTRS{idProduct}=="1436", NAME="phone", MODE="0666",SYMLINK+="phone"
KERNEL=="ttyUSB*", ATTRS{idVendor}=="067b", ATTRS{idProduct}=="2303", NAME="dylos", MODE="0666",SYMLINK+="dylos"