contoh yg pernah saya coba dan berhasil, semoga membantu:

#include <SoftwareSerial.h>
SoftwareSerial mySerial(3,4); // RX=3, TX=4

void setup() {

    mySerial.begin(9600);

//test kirim SMS pake modem wavecom fastrac
    mySerial.println("at+cmgs=089630423432");
    mySerial.write(13);
    delay(1000);
    mySerial.println(mssg);
    mySerial.write(26);
 
}

void loop
{
}