Kristof,
Sorry, do not think that K750 will work that easy but the T68i should do so.
I suggest below connection!
Pin 4: Rx
Pin 5: Tx
Above information found at: Sony Ericsson R520, R310, R320, T28, T39, T68, T68i, T610, T630 and others cell phones cable connector pinout and wiring @ old.pinouts.ru