TTGO T-Call SIM800L V1.4 — How to Fix Audio Issues

#define MODEM_RST       5
#define MODEM_PWKEY     4
#define MODEM_POWER_ON  23
#define MODEM_TX        27
#define MODEM_RX        26
 
void setup() {
  Serial.begin(115200);
 
  pinMode(MODEM_PWKEY, OUTPUT);
  pinMode(MODEM_RST, OUTPUT);
  pinMode(MODEM_POWER_ON, OUTPUT);
  digitalWrite(MODEM_PWKEY, LOW);
  digitalWrite(MODEM_RST, HIGH);
  digitalWrite(MODEM_POWER_ON, HIGH);
 
  Serial1.begin(115200, SERIAL_8N1, MODEM_RX, MODEM_TX);
  delay(6000);
 
  Serial.println("Ready. Monitor: 115200, 'Both NL & CR'. Type commands.");
}
 
void loop() {
  if (Serial.available()) Serial1.write(Serial.read());
  if (Serial1.available()) Serial.write(Serial1.read());
}