incomingCallNumber issue with GSM/GPS shield

hi folks,
I have a problem with the fona library and my SIM5320 GSM/GPS shield (w Arduino UNO).
I have managed to connect and to send receive SMS but I still have some issues with std calls.

What I want to do is simply recognize the caller id number.

I have found in the Adafuit_FONA.cpp file a specific function for doing this, which is “incomingCallNumber”

here’s the code of part of the file:

void Adafruit_FONA::onIncomingCall() {

  DEBUG_PRINT(F("> ")); DEBUG_PRINTLN(F("Incoming call..."));

  Adafruit_FONA::_incomingCall = true;
}

boolean Adafruit_FONA::_incomingCall = false;

boolean Adafruit_FONA::callerIdNotification(boolean enable, uint8_t interrupt) {
  if(enable){
    attachInterrupt(interrupt, onIncomingCall, FALLING);
    return sendCheckReply(F("AT+CLIP=1"), ok_reply);
  }

  detachInterrupt(interrupt);
  return sendCheckReply(F("AT+CLIP=0"), ok_reply);
}

boolean Adafruit_FONA::incomingCallNumber(char* phonenum) {
  //+CLIP: "<incoming phone number>",145,"",0,"",0
  if(!Adafruit_FONA::_incomingCall)
    return false;

  readline();
  while(!prog_char_strcmp(replybuffer, (prog_char*)F("RING")) == 0) {
    flushInput();
    readline();
  }

  readline(); //reads incoming phone number line

  parseReply(F("+CLIP: \""), phonenum, '"');


  DEBUG_PRINT(F("Phone Number: "));
  DEBUG_PRINTLN(replybuffer);


  Adafruit_FONA::_incomingCall = false;
  return true;
}

unfortunately when I call this function in my script nothing happens.
What I preventively did was to Enable incoming call notification (see my code)

in order to have some output in the form //+CLIP: "<incoming phone number>",145,"",0,"",0 as displayed in the .cpp file, but still no information is received…

I am also attaching my code in order for you to understand

#include "Adafruit_FONA.h"

#define FONA_RX            11
#define FONA_TX            10
#define FONA_RST           5

#include <SoftwareSerial.h>
SoftwareSerial fonaSS = SoftwareSerial(FONA_TX, FONA_RX);
SoftwareSerial *fonaSerial = &fonaSS;
Adafruit_FONA_3G fona = Adafruit_FONA_3G(FONA_RST);

void setup() {
  Serial.begin(4800);
  Serial.println(F("FONA incoming call example"));
  Serial.println(F("Initializing....(May take 3 seconds)"));

  fonaSerial->begin(4800);
  if (! fona.begin(*fonaSerial)) {
    Serial.println(F("Couldn't find FONA"));
    while(1);
  }
  Serial.println(F("FONA is OK"));

  // Enable incoming call notification
  Serial.println(F("Enabling incoming call notification..."));
  fona.sendCheckReply(F("AT+CLIP=1"), F("ok!"));

void loop(){
  // Create a small string buffer to hold incoming call number.
  char phone[32] = {0};
  // Check for an incoming call.  Will return true if a call is incoming.
  if(fona.incomingCallNumber(phone)){
    Serial.println(F("RING!"));
    Serial.print(F("Phone Number: "));
    Serial.println(phone);
  }
}

I have to admit that I have copied this code from link
and I do not still understand the purpose of wiring the interrupt PIN of that script.

thanks for the help