I think they've sent >100,000 faxes? (Based on https://www.recode.net/2017/4/27/15440440/voters-text-pages-fax-political-views-protest-resistbot-trump-health-care from April, presumably substantially more since then).
Tech stach is Python, Flask, RapidPro, AWS.
Info: https://resistbot.io/about.html