I think the signals might be the wrong way up for the connection directly to the Arduino. (as opposed to the computers RS232 line)
The data sheet says:-
Rx (pin5)
The computer's RS232 transmit signal is directly
connected to this pin through a single current
limiting resistor (typically about 47KW). Internal
signal inversion and Schmitt trigger waveshaping
provide the necessary signal conditioning.
This means if you are feeding the TX from an Arduino the signal will be upside down as the Arduino produces a logic one to indicate mark but the chip is expecting a logic zero.
The data sheet is a little more ambiguous about pin 6 but that should be all right.