Arduino response USSD command truncated

Question

I'm trying to send a USSD string with Arduino/SIM900 module. This is my code:

mySerial.println("AT+CMGF=0");
   delay(100);

   mySerial.println("AT+CUSD=1,\"*123#\"");
   delay(10000);
if (mySerial.available() >0)
  {

    while(mySerial.available() > 0)
    {
      incoming_char=mySerial.read(); //Get the character from the cellular serial port.

      Serial.print(incoming_char);

    }
 }

the problem is that, the response is truncated. If I try to remove the row

mySerial.println("AT+CMGF=0");

the response is more long but always truncated

How to resolve the problem?

Thanks a lot