mgetty don't launch pppd on incming call. This is due to compilation options of the package, that do not include the -DAUTOPPP. I've compiled the package manually with this option and now, my ppp server is working fine. I've wasted about 3 days, tested numerous modem, lines and config before finding the source of the bug.
*** Bug 8465 has been marked as a duplicate of this bug. ***
Updated, I have no way of testing this, so if my fix isn't right please let me know.