SIM800L V2.0 with FONA library - getting caller number and compare

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Hi,
I want to build a call remote for a pre heating.
SMS isn't that good for the use because my friend who will use this doesn't have a SMS flatrate .

So - I'm using a SIM800L v2.0 module and got it working with the fona library.

I can call the remote, it switches on the pre heating and if the status led of the pre heating is on it will return a beep via DTMF. If not it will return three beeps via DTMF.

Now I wanted to filter the callers, so the remote have to pickup only when one number out of a list (inside sketch or sim card, sim card is better) is calling. If the number is invalid it should not pickup.

I tried to get the number with "fona.incomingCallNumber()" but it won't show.
Also there's no +CLIP output on serial monitor when I'm calling - but again, the SIM800L / Arduino recognizing a call and pick up the phone.

The FONA library needs the RING pin but my SIM800L Module doesn't have any. So I soldered a wire to a test pin on the board - see this pic:

After looking on a circuit diagram and follow the pinout with my multimeter it should be the same as the RING pin on the original FONA module - it's directly connected to PIN68 of the SIM800 chip.

I hope it's clear what I want to do and I also hope that someone can help me with this.
I've searched for 5 days and found nothing but SMS sketches.

With the FONA test sketch from github the serial monitor shows the AT+CLIP output and I don't know why my sketch won't.

Here's my working sketch WITHOUT number recognition:

#include "Adafruit_FONA.h"


#define FONA_RX 11
#define FONA_TX 10
#define FONA_RST 12
#define FONA_RI_INTERRUPT 0
// no RING pin on SIM800L V2.0 module but I soldered a cable to a test pin which is on the way to the RING LED.
#define BAUD 9600



int heizung = 7;
int heizung_status = 6; 
int led = 13; // just a testing led on my breadboard

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

Adafruit_FONA fona = Adafruit_FONA(FONA_RST);


int callstat;

void setup() {
  Serial.begin(9600);
  fonaSerial->begin(4800);
  if (! fona.begin(*fonaSerial)) {
    Serial.println(F("Couldn't find FONA"));
    while (1);
  }

  pinMode(heizung, OUTPUT);
  pinMode(heizung_status, INPUT_PULLUP);
  pinMode(led, OUTPUT);


}



void loop() {

  callstat = fona.getCallStatus();
  switch (callstat) {
    case 0: Serial.println(F("Bereit.")); break; 
    case 1: Serial.println(F("Status unbekannt")); break;
    case 3: Serial.println(F("Es klingelt")); //Ringing - @forum: Output works!
            delay(5000); // 5 seconds delay
            fona.pickUp(); // pickup phone - @forum: works!
            break; // Case-Ende
    
    case 4: Serial.println(F("Anruf aktiv"));  //Anruf angenommen...
            delay(2000); // 2 seconds delay
            digitalWrite(heizung, HIGH); // set heater switch to high
            
            
            if (digitalRead(heizung_status)) { // if input 6 is high, beep three times an hangup - high means "not good" @ forum: NPN will pass GND to PIN6 if good.
              Serial.println(F("Heizung ist NICHT an!")); // print that the heater is off
              digitalWrite(led, HIGH); // just a stauts LED for testing
              fona.println("AT+VTS=9"); // send DTMF beep for number 9
              delay(800);
              fona.println("AT+VTS=0"); // send DTMF beep for number 0
              delay(800);
              fona.println("AT+VTS=9"); // send DTMF beep for number 9
              delay(2000);
              digitalWrite(led, LOW); // set status LED to low, even it should be low.
              delay(5000);
              fona.hangUp(); // as it says, hangup.
            }
            if (digitalRead(!heizung_status)) { // if input 6 is low, beep just once and hangup.
              digitalWrite(led, LOW); // set status led to low, just for testing.
              Serial.println(F("Heizung ist AN!")); // print that the heater is on
              fona.println("AT+VTS=2"); // send DTMF beep for number 2
              delay(1000);
              digitalWrite(heizung, LOW); // set heater switch to low
              delay(2000);
              fona.hangUp(); // as it says, hangup.
            }
 
            break;

            
    default: Serial.println(F("Unbekannt"));
  }


}