Problem with Arduino Uno and SIM900 shield logical communication (rx & tx) The logical communication(rx &amp; tx) between the shield and Arduino Uno doesn't  work. Arduino (rx &amp; tx) supply  is 0 &amp; 5 V but this is 0 &amp; 2.8 V. What am I to do?
Use a logic level converter.

A logic level converter is a small device that converts between different logic levels, for example between 5V and 3.3V.

I am not affiliated with Sparkfun, I just think they are the most informative source.