we are also doing same kind of project
if u take nokia the logic is ttl ,like that
if u see the fbus circuit diagram , u see a 7805 and max232 for transfer to cmos logic
they use +12 from your CTS of com port
if u use the cable and program with VB or ASM(for mcu) u wont get any response , but if u restart the phone ie switch off and on u will get some code.
in the case of erricson used in riccibitti.com it is working , may be directly compactble , ie. , phone used for communicating using your own program.
if u use nokia pc suite for your phone it will work
for better reason and idea for it contact my project partner giri
he will explain where is the problem clearly