wemos d1 mini keeps resetting while using SIM800L

Question

I am trying to do a small project with SIM800L with Wemos d1 mini. It is a DTMF based project. The problem here is, every time I make a call the d1 mini is getting reset. This is my first project with d1 mini. For the past few days I have googled, trying to find a solution. But I failed. I will be very grateful if you can provide the solution.

I am getting these errors in the serial monitor:

 ets Jan  8 2013,rst cause:4, boot mode:(3,7)
wdt reset
load 0x4010f000, len 3460, room 16 
tail 4
chksum 0xcc
load 0x3fff20b8, len 40, room 4 
tail 4
chksum 0xc9
csum 0xc9
v00043b40
~ld

I am using Arduino IDE. Version: 1.8.8

Here is my project code.

#include <SoftwareSerial.h>
#define rxPin D3
#define txPin D4
SoftwareSerial sim800L(rxPin, txPin);
#define RELAY_1 D7
#define RELAY_2 D8
boolean relay1_state = false;
boolean relay2_state = false;
//stores incomming data from sim800l
String buff;
String dtmf_cmd;
boolean is_call = false;
String ownerNumber = "xxxxxxxxxxxxxxx";
void setup()
{
  pinMode(RELAY_1, OUTPUT);
  pinMode(RELAY_2, OUTPUT);
//Begin serial communication with (Serial Monitor)
  Serial.begin(115200);
//Begin serial communication with (SIM800L)
  sim800L.begin(9600);
  Serial.println("Begin serial communication with (SIM800L)");
  delay(500);
sim800L.println("AT"); //send AT
  delay(500);
sim800L.println("AT+DDET=1"); //Enable DTMF
  delay(500);
sim800L.println("AT+CLIP=1"); // Enable CallerID
  delay(500);
}
void loop()
{
  while (sim800L.available()) {
    buff = sim800L.readString();
    Serial.println("Raw: " + buff);
if (is_call == true) {
  if (int index = buff.indexOf(&quot;+DTMF:&quot;) &gt; -1 ) {
    index = buff.indexOf(&quot;:&quot;);
    dtmf_cmd = buff.substring(index + 1, buff.length());
    dtmf_cmd.trim();
    Serial.println(&quot;dtmf_cmd: &quot; + dtmf_cmd);
    doAction();
  }
  if (buff.indexOf(&quot;NO CARRIER&quot;) &gt; -1) {
    sim800L.println(&quot;ATH&quot;);
    is_call = false;
  }
}
if (buff.indexOf(&quot;RING&quot;) &gt; -1) {

  // Now check the caller number
  if (buff.indexOf(&quot;+CLIP:&quot;)) {
    unsigned int index, index1;

    index = buff.indexOf(&quot;\&quot;&quot;);
    index1 = buff.indexOf(&quot;\&quot;&quot;, index + 1);

    String temp = buff.substring(index + 2, index1);
    temp.trim();

    if (ownerNumber == &quot;+&quot; + temp) {
      Serial.println(&quot;Number matched&quot;);
      delay(2000);
      sim800L.println(&quot;ATA&quot;);
      is_call = true;
    } else {
      Serial.println(&quot;Number not matched&quot;);
    }
  }
}

}
while (Serial.available())  {
    sim800L.println(Serial.readString());
  }
}
void doAction() {
  if (dtmf_cmd == "1") {
    relay1_state = !relay1_state;
    digitalWrite(RELAY_1, relay1_state);
    if (relay1_state == true)
    {
      Serial.println("Relay 1 has been ON");
    }
    else
    {
      Serial.println("Relay 1 has been OFF");
    }
  }
  else if (dtmf_cmd == "2") {
    relay2_state = !relay2_state;
    digitalWrite(RELAY_2, relay2_state);
    if (relay2_state == true)
    {
      Serial.println("Relay 2 has been ON");
    }
    else
    {
      Serial.println("Relay 2 has been OFF");
    }
  }
}