String smsString, Ontvanger, testmsg, testnumber;
void setup() {
  Serial.begin(115200);
  smsString = String("Test");
  Ontvanger = String("+3161234567");
  testmsg = String("Test");
  testnumber = String("+3161234567");
}

void loop() {

  if  ((smsString == testmsg) && (Ontvanger == testnumber) )
  {
    Serial.println("Relay Aan");
    delay(1000);
    Serial.println("Relay Uit");
  }

  else
  {
    Serial.println("Does not work");
  }

  delay(1000);
}