Limit outbout lines in fax

 I have a card with 4 ports fx0 that use it to connect to 4 analog lines to call the street.

I want to operate the fax server (hylafax) but do not want the fax server (hylafax) occupy the 4 lines, because I would leave no possibility of making calls abroad.

how i can configure fax server only use one of my 4 lines?

Hylafax use a IAX extension.

thanks in advance.