Kalau punya USB to Serial (misal FTDI232, atau set Arduinonya jadi gitu*) bisa dicoba kirim/terima langsung AT command/~response ke/dari sim900/komputer, pakai program Putty. Awas tegangan. * misal UNO: http://www.instructables.com/id/Turn-Arduino-into-USB-to-TTL-Adapter-with-1-wire/