Default pin settings of the Adafruit Fona library are not working on the ESP32. User mobimation found a solution.

Following changes are necessary.

#define FONA_RX 2
#define FONA_TX 3
#define FONA_RST 4

needs to be

#define FONA_RX 9
#define FONA_TX 8
#define FONA_RST 4
#define FONA_RI 7