VOH = 2.7V (for SIM800l): It means that this is the minimum voltage that the SIM800L Module will assert as Logic High (Output).
VIH = 3.3V (for ATmega328P): It means that this is the minimum voltage level that ATmega328P MCU will reliably detect as Logic HIgh (Input).
As 2.7V (VOH) of SIM800L is less that 3.3V (VIH) of ATmega328P, their direct interfacing will not work. The solution is to use level sifter of Fig-1.
Figure-1;