Few questions that come to our mind while interfacing are,
Does it work if we connect the SIM300 UART lines directly to microcontroller operating at 5V?
Yes, even though the SIM300 specifications say that its maximum operating voltage is 4.5V, SIM300 UART lines work with microcontrollers operating at 5V. You can connect your SIM300 GSM modem TX and RX lines directly to 5V microcontrollers TX and RX lines. Risk is involved in this, as we are exceeding the recommended ratings of SIM300. But it does work in the projects i have designed so far.
What else can we do to safely use SIM300 GSM modem with a 5V microcontroller?
If you want to play safe, you can use two MAX232 RS232 line drivers. The first MAX232 to convert from GSM voltage levels to RS232 voltage levels. Second MAX232 to convert from RS232 voltage levels back to microcontroller voltage levels.
You can also use dual supply voltage level translators available to translate voltage levels.