Well, if we talk about goverbment whistleblower programs I assume using a fax from a random location while travelling would be good enough to not be identified by your employer (or whom ever you blow whistle on).

If you do blow the whistel on the government, well, you better trust your journalist partner a lot. Or use a trustworthy lawyer as a go between.

But hey, one could send the fax back and forth, cut of the fax numbers and physically mail it from a very busy train station! And have a someone put the letter into the letterbox for you, someone you pay out of view of any CCTV cameras!