Which board do you use? A -050? Where do you connect the SIM900? It has a TTL-Level UART I think, so don't connect it to the RS232 port on that board. Instead connect it to the PSoC pins directly,
Additionally it might be required to send a <CR> (0x0d) to tell the modem that the line ended. Watch for an "OK" returned.
it still doesn't work even with a \r at the end
I am not connecting using the RS232 port , I am connecting it directly to the pins.
Let us have a look at your project with all of your settings. Please use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.
At the end of the team should be Symbol ';'
UART_PutString ("ATD + 7910XXXXXXX; \ x0d");
Be sure to connect the battery, otherwise the device can freezes after a call.
Perhaps the project for SIM900 & PSoC 4 will help you. It here.
Another thing, if you are connecting directly to an RS232 line driver on the other end, you may have to put inverting (NOT) buffers on the output and the input of your RS232 unit, between the Pin and the RX, and the Pin and the TX.
RS232, when going outside of your device, to another RS232 line driver, usually requires inversion of the signal.