SIM800C not responding during call and few seconds after call

Hi,

I have this board from ebay SIM800C Development Board Quad-band GSM GPRS Bluetooth Module w/Antenna inm 6216809757456 | eBay

When I type ATDsomeNumber;\r\n I can't read anymore from serial port... trying to send multiple times simple AT. After call is automatically dropped there is delay about 5 seconds till GSM module starts responding.

My PSU is 12V 10A converted into 5V 3A. 5V supplies ESP32, RFID and GSM module. USB is still connected to ESP32 for debug reasons. There should be enough power.

I also tried to configure sleep mode to 0. No change at all :frowning:

Any idea how to resolve this issue :confused: ?

Thanks