In the mean while i discovered that the K750i doesn't have a straight serial connection. (I tried everything) When installing the phone in windows it creates a few virtual com ports. So it's not possible to connect the arduino to the phone straight away.
The only thing you can use is maybe this:
http://www.circuitsathome.com/products-page/arduino-shields/usb-host-shield-for-arduino/
I now bought an old T68i. It came with a KRY 105 192 R2C “USB docking station” :
http://cgi.ebay.co.uk/Sony-Ericsson-KRY-105-192-R2C-KRY-111-117-R6A-Cable-/150544506221
After finding the right driver for this and installed in in xp I saw 1 serial com port. This was the usb to rs232 com port from the docking station. I connected the phone to the docking station, opened HyperTerminal, choose the com port of the docking station and I can send at commands. Later this day I'm going to buy a DCU-11 cable, I will remove the usb to serial converter and will test it when connected the arduino to the T68i. I think I need to use a voltage divider:
I will keep you posted.