Description 
The SIM900A is a readily available GSM/GPRS module used in many mobile phones and PDA. The module can also be used for developing IOT (Internet of Things) and Embedded Applications. SIM900A is a dual-band GSM/GPRS engine that works on frequencies of EGSM 900MHz and DCS 1800MHz. SIM900A features GPRS multi-slot class 10/ class 8 (optional) and supports the GPRS coding schemes CS-1, CS-2, CS-3 and CS-4.
 
SIM900A GSM  Features
Single supply voltage: 3.4V – 4.5V
Power saving mode: Typical power consumption in SLEEP mode is 1.5mA
Frequency bands:SIM900A Dual-band: EGSM900, DCS1800. The SIM900A can search the two frequency bands automatically. The frequency bands also can be set by AT command.
GSM class: Small MS
GPRS connectivity:GPRS multi-slot class 10 (default) , GPRS multi-slot class 8 (option)
Transmitting power: Class 4 (2W) at EGSM 900, Class 1 (1W) at DCS 1800
Operating Temperature: -30ºC to +80ºC
Storage Temperature: -5ºC to +90ºC
DATA GPRS: download transfer max is 85.6KBps, Upload transfer max 42.8KBps
Supports CSD, USSD, SMS, FAX
Supports MIC and Audio Input
Speaker Input
Features keypad interface
Features display interface
Features Real Time Clock
Supports UART interface
Supports single SIM card          
Firmware upgrade by debug port
Communication by using AT commands
 
Resources: 
Project Example
Tutorial to Interface GSM SIM900A With Arduino