A quick HN search (<a href="http://www.hnsearch.com/search#request/all&q=fax+api" rel="nofollow">http://www.hnsearch.com/search#request/all&q=fax+api</a>) also surfaced Phaxio, RingCentral, and PamFax.<p>My personal recommendation would be to check out PamFax + Tropo (a competitor to Twilio) which have partnered up. More info at <a href="http://www.pamfax.biz/en/partners/tropo/" rel="nofollow">http://www.pamfax.biz/en/partners/tropo/</a> and <a href="http://blog.tropo.com/2011/02/21/send-a-fax-with-your-voice/" rel="nofollow">http://blog.tropo.com/2011/02/21/send-a-fax-with-your-voice/</a>