NOCHARGE with MFCR2 signalling


I´m new here and I´m trying to get around with Asterisk. I have posted my problem in another forums but till now I´ve got no response. Well, my problem is the following: I´ve managed to install Asterisk with MFCR2 support successfully but I would like to know how can I change the CHARGE signal that defines that the calling will generate a billing. That is, I´m trying to make Asterisk send a NOCHARGE signal instead of a CHARGE signal to all callings terminating on it. Is this possible with the MFCR2 protocol? Do I need to change any files like chan_unicall.c or unicall.c and compile them again? Thanks in advance!!