One alternative is hylafax+ (or hylafax) and
but you would need to install iaxmodem's and/or t38modem's to connect asterisk to hylafax and a little RTFM'ing which I guess for many is the downside. The upside is it's a lot cheaper and perhaps a little more functional/robust and will eventually deliver most faxes piecemeal even if the first attempt fails part way through, even with a less than "very good" network 