SIM800L EVB audio output on Raspberry Pi
You are not missing anything. The functionality exists on the SoC (SIM(00L)) but is not connected to any external pins on some of the models.
You need to buy a SIM800L module with audio connected from the SoC to external pins.
Ref.: Datasheet
These are the pins on the underside of the metal capsule (SIM800L SoC.)