Problem connecting Arduino Uno R3 to SIM900 GPRS/GSM shield and sending an SMS - Arduino GSM Shield - Arduino Forum
Sim900/sim900A Mini Module With Arduino UNO The Arduino IDE includes a library for their GSM shield using the Quectel M10 GSM chip, and Surprise! it is almost identical to the Sim900A chip!
SIM900 GSM GPRS Shield with Arduino | Random Nerd Tutorials
Arduino Sim900a Gsm Module Interfacing With Arduino Uno | Arduino