No byte on my Rx uart port

[nicola]

HI all,
I have this esp32 board : https://github.com/Nicholas3388/LuaNode

and I'm interfacing the uart2 (pin 17, pin 16) with this sim800l module :
https://www.amazon.com/SIM800L-Quad-ban ... B017I6QC8C

on my esp32 I try to make a loopback test by shorting Tx and RX and I get correctly what I transmit on the Tx line (GPIO17) back to my Rx line (GPIO16).

Now I have connected the esp32 board with the sim800l Tx-Rx pair (by crossing them like attached). The SIM800 pins should have voltage level pin from 2.7V to 2.8V reading the Sim800 datasheet.

When I call the sim800 I expected the "Ring" string on the Rx pin of my esp32 but, even if the module is correctly registered to the network, I get no byte in the esp32.

Same behaviour even if I send the standard "AT\r\n" command form the Tx line of my ESP32 board.

I wonder if :

Is 2.7-2.8 Volts good for the Rx pin of the Esp32 ?
Is there any problem by using these pin of my esp-Wroom-32 board (maybe they are shared with other peripherals)?

Hope you can help.
thanks a lot.
Nicola