You can do this without an application by echoing commands to the modem's serial port (ttyS18) while running
get the results back.
But I recommend using minicom, an application specifically designed to talk to serial modems at the AT command level. At the least, you can use this to interactively test a sequence of AT commands to see if what you get is the expected result or not, before scripting them.
Chat is another program that might be useful to you; it lets you set up a “conversation” with your modem, telling it which AT commands to give, what results to expect, and what action to take when it gets a wrong result. It will also let you hand over the SMS message to another program, eg. to save it to a file or email it to you.