Arduino UNO | ICOMSAT v1.1 SIM900 GSM Module


I am trying to use UNO with ICOMSAT v1.1 SIM900 GSM Module . I am currently trying with this GitHub - jgarland79/GSM_Shield: GSM Library for the SIM900 chip and the GBoard Arduino GSM development board library . I have set the rx,tx pin as 2,3 and have also connected uno to external supply but i am not getting any response from the GSM module . I send a simple AT command but get no response.I think UNO is not able to communicate with the GSM . What change should i do ? Another Library?