Why does my USB modem produce multiple /dev/ttyUSB* files?

I have a single Sierra Wireless USB 308 modem we are using for sending SMS message to, and receiving SMS messages from, customers. When this single device is plugged in, the system creates multiple device files: /dev/ttyUSB0, /dev/ttyUSB1, /dev/ttyUSB2, /dev/ttyUSB3. I read somewhere that the sending of SMSes must use the third device file (/dev/ttyUSB2). I'm using SMS tools . Questions: 1. Why does a single device produce multiple device files in the /dev directory? 2. I want to be able to symlink /dev/ttyUSB2 to /dev/sms-sierra. When running udevadm info -a /dev/ttyUSBXXX on each device file (where "XXX" is "0", "1", "2" or "3"), the information for each "device" is almost identical. Because the device numberings are not always the same, how can I write a udev rule to detect the appropriate device number to use? For question 1, I have no idea. For question 2, is there a program required in order to probe the device file as to whether it can send SMS (e.g., by seeing if it responds to the appropriate modem "AT" commands)?