added support for USSD commands

Author: cristidragomir97

Diff for: examples/ReceiveSMS/ReceiveSMS.ino

@@ -52,6 +52,12 @@ void setup(){
     cellular.connect();
     Serial.println("Connected!");
 
+    // Send USSD command for cost control, if supported by the network provider
+    // Some network providers send the response as an SMS message
+    // Some network providers send the response as a USSD response
+    //Serial.println("Sending USSD command...");
+    //Serial.println(cellular.sendUSSDCommand("*123#"));
+
     // Register interrupt based callback for new SMS
     attachInterrupt(digitalPinToInterrupt(NEW_SMS_INTERRUPT_PIN), onSMSReceived, RISING);
 

Diff for: src/ArduinoCellular.cpp

@@ -363,6 +363,10 @@ std::vector<SMS> parseSMSData(const String& data) {
   return smsList;
 }
 
+String ArduinoCellular::sendUSSDCommand(const char * command){
+    return modem.sendUSSD(command);
+}
+
 std::vector<SMS> ArduinoCellular::getReadSMS(){
     String rawMessages = sendATCommand("+CMGL=\"REC READ\"");
     if(rawMessages.indexOf("OK") == -1){

Diff for: src/ArduinoCellular.h

@@ -184,6 +184,9 @@ class ArduinoCellular {
          */
         String sendATCommand(const char * command, unsigned long timeout = 1000);
 
+        String sendUSSDCommand(const char * command);
+    
+
 
         /**
          * @brief Gets the Network client. (OSI Layer 3)