SIP fax sending software?

Can anyone recommend software that sends faxes over SIP? I have plenty of inbound fax to email services, but now and then I need to send a reply and it looks tacky to use one of the free web ones that put an ad on it. I know that if I wanted to pay $15/mo there are lots of lovely services but we're taking about one fax a month, maybe, here. Ideally it'd take a postscript or PDF or Word document and a phone number and fax it to that number. I have Ubuntu, FreeBSD, and MacOS boxes. Any suggestions? Regards, John Levine, johnl@iecc.com, Primary Perpetrator of "The Internet for Dummies", Please consider the environment before reading this e-mail. https://jl.ly