Can't read sms using Thinker A7 communicate with Arduino

Official Hardware Retired GSM Shield
1

Hi guy

I'm trying to communicate with the Ai Thinker A7 module. On Arduino Uno R3 Im using pin 7,pin 8 to connect with UTXD & UAXD. When i send sms from my phone to module thinker 7, i use arduino to read SMS.
My code as bellow

#include <SoftwareSerial.h>
#include <string.h>
char incoming_char = 0;
SoftwareSerial cell(7, 8); //my shield uses these pins for software serial.

void setup()
{
 Serial.begin(115200);
 cell.begin(115200);
 delay(2000);
 Serial.print("Starting modem communication...");
 delay(12000);
 Serial.print("OK\nIntroduce your AT commands:\n"); 
}

void loop()

{
 if(cell.available() > 0)
 {
   incoming_char=cell.read();
   
   if(incoming_char=="a"){
     Serial.println("Content of SMS is a"); //If the SMS contains "#" the arduino looks for commands that follow.
     Serial.println(incoming_char);

   }
   else
   {
      Serial.println("Difference a");
      Serial.println(incoming_char);

   }
 }
}

But i can't read sms when my phone send sms. I check on Serial Monitor. Result same picture attached as bellow

Please help me fix it. Thanks a lot