SIM800L problems reading from serial

Hi guys, this is my first question in arduino forum,
I'm trying to connect SIM800L (my first time using GSM/GPRS protocols) but I have faced a problem
I made a simple softwareSerial code to read and write from serial, without sending any AT command the serial print infinite amount of question mark!

For the information I powered the chip 4.2 volts and it kept blinking for 1 sec (7 times the stops for the 8th)

When I powered it to 6v 2A the led indicator shows it's connected to the SIM card then the ? Appears in serial monitor

I checked the voltage from arduino Tx it sends 2.8 v to the chip

After uploading a calling code to my SIM800L the regular ? Showed but it did make the call!!

Any help please?

I made a simple softwareSerial code to read and write from serial, without sending any AT command the serial print infinite amount of question mark!

You didn't post the code, but you want us to explain why it does what it does. How is that supposed to work?