What is the card insertion direction of a6 mini gsm module?

Make sure the wires are connected correctly. Tx, rx RX should cross.

Ensure that the power supply is good, and the instantaneous current of this module will be relatively large when making a phone call.

Whether the serial port level of single chip microcomputer matches the module level.

If you doubt the baud rate, just set the MCU to the same baud rate as the serial debugging assistant. . . .

If it still doesn't work after changing the baud rate, directly find a usb ttl serial port rx to monitor the tx and rx of the module after MCU connects A6 to see if there is any data. If yes, see A6 module's reply to the AT command you sent.

I don't know if you are using a module with 232 and ttl serial port. I just looked at it, and that module is a bit guilty. I directly connect the two outputs to rxd of the module, which is obviously better than MCU and 232. . . . . A little guilty. Or its schematic diagram is different from the actual one, which circumvents this problem in other ways.