I'm not sure if there is such a AT command but anyway you can find all the sim900 AT command in the link below. https://www.robotshop.com/media/files/pdf/command-manual-im140318007.pdf
I'm not sure if there is such a AT command but anyway you can find all the sim900 AT command in the link below. https://www.robotshop.com/media/files/pdf/command-manual-im140318007.pdf