Accessing the GSM modem on a linux mobile (eg. N900)

Question

I'm trying to access the GSM modem of a mobile set, so I can send AT commands to it. Instead of programming, I tried to have a test run using minicom on the phone. The problem is I cannot find the device I should connect to, I tried /dev/tty0 tty1 and also ttyS0 and alike, but none of them responded to my AT command.

So what should I do? Is it even possible?

Platform: Nokia N900 (running maemo linux)