the code above works with my SIM800L module.
not all is necessary, but maybe helpful …
what I have learned up to now:
it works with HardwareSerial; no libraries to declare; default pins
I can make phone calls
I can send and receive SMS
the audio of the phone is not connected to the M5Stack; it's just the bare minimum.
if required, I might connect a better antenna - there is a connector on the SIM800L module and a hole in the base module I could use.
no soldering, no cables, no voltage problems
no EDGE, no LTE
the LED on the SIM800L module can be seen through the headphone hole.
the Arduino IDE installation brings a GSM library, but it is not listed under examples in the IDE.
bye for now