A quick HN search (http://www.hnsearch.com/search#request/all&q=fax+api http://www.hnsearch.com/search#request/all&q=fax+api) also surfaced Phaxio, RingCentral,

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