I've got an old nokia 6210 phone working with serial (DB9) cable sucefully working with serial terminal in the PC throughout AT commands (sending sms, read sms, ect.). When linking RX/TX/GND from the DB9 to the arduino i got no response in any serial commands. Any ideas? I've been reading about TTL levels and MAX232 converters, would this aply to this?
If you could connect it directly to a Pc serial port with the DB9 connector, then yes you are right you need the levelshifter to make it compatible with Aduinos serial port.