i find the problem because the software serial on sim900 work only on arduino uno (7,8 pins ) and on arduino mega work at TX RX pins now i connect on mega (tx2,rx2) and work good. thank you guys for support 
i find the problem because the software serial on sim900 work only on arduino uno (7,8 pins ) and on arduino mega work at TX RX pins now i connect on mega (tx2,rx2) and work good. thank you guys for support