Can you get a MAX232 and built a RS232/TTL converter?
Like I said you need the converter.
Build this and connect the TTL part to your arduino UART port and the RS232 to you modem.
It will work then
http://www.google.pt/imgres?imgurl=http://www.maximintegrated.com/images/qv/1798.gif&imgrefurl=http://www.maximintegrated.com/datasheet/index.mvp/id/1798&h=514&w=672&sz=21&tbnid=xRhYXcxZzeed-M:&tbnh=90&tbnw=118&zoom=1&usg=__cg1Hy6Y45imU2dfpK3MQQ7DKygQ=&docid=SseYPf5Fi5CmLM&sa=X&ei=DH-pUv_HAoqSswbp_4CgDQ&sqi=2&ved=0CDsQ9QEwAA